diff --git a/backend/shared/db/__tests__/auditColumns.test.ts b/backend/shared/db/__tests__/auditColumns.test.ts
new file mode 100644
index 00000000..42465114
--- /dev/null
+++ b/backend/shared/db/__tests__/auditColumns.test.ts
@@ -0,0 +1,116 @@
+import { AuditContext, withAuditColumns, AUDIT_TABLES } from '../auditColumns';
+
+describe('AuditContext', () => {
+  it('creates from user request', () => {
+    const ctx = AuditContext.fromRequest({ userId: 'user-123' });
+    expect(ctx.actorId).toBe('user-123');
+    expect(ctx.actorType).toBe('user');
+  });
+
+  it('creates from nested user object', () => {
+    const ctx = AuditContext.fromRequest({ user: { id: 'user-456' } });
+    expect(ctx.actorId).toBe('user-456');
+    expect(ctx.actorType).toBe('user');
+  });
+
+  it('creates from API key request', () => {
+    const ctx = AuditContext.fromRequest({ apiKey: { id: 'key-789' } });
+    expect(ctx.actorId).toBe('key-789');
+    expect(ctx.actorType).toBe('api_key');
+  });
+
+  it('defaults to system when no actor is present', () => {
+    const ctx = AuditContext.fromRequest({});
+    expect(ctx.actorId).toBe('system');
+    expect(ctx.actorType).toBe('system');
+  });
+
+  it('system() creates a system context', () => {
+    const ctx = AuditContext.system();
+    expect(ctx.actorId).toBe('system');
+    expect(ctx.actorType).toBe('system');
+  });
+
+  it('anonymous() creates an anonymous context', () => {
+    const ctx = AuditContext.anonymous();
+    expect(ctx.actorId).toBe('anonymous');
+    expect(ctx.actorType).toBe('system');
+  });
+});
+
+describe('withAuditColumns.insert', () => {
+  it('builds INSERT with created_by and updated_by', () => {
+    const ctx = new AuditContext('user-123', 'user');
+    const { sql, params } = withAuditColumns.insert(
+      'subscriptions',
+      { user_id: 'u1', name: 'Pro', amount: 99 },
+      ctx,
+    );
+
+    expect(sql).toContain('INSERT INTO subscriptions');
+    expect(sql).toContain('created_by');
+    expect(sql).toContain('updated_by');
+    expect(params).toEqual(['u1', 'Pro', 99, 'user-123', 'user-123']);
+  });
+
+  it('generates correct placeholder numbering', () => {
+    const ctx = AuditContext.system();
+    const { sql } = withAuditColumns.insert(
+      'users',
+      { email: 'a@b.com', name: 'Alice' },
+      ctx,
+    );
+    expect(sql).toContain('$1');
+    expect(sql).toContain('$2');
+    expect(sql).toContain('$3');
+    expect(sql).toContain('$4');
+  });
+});
+
+describe('withAuditColumns.update', () => {
+  it('builds UPDATE with updated_by', () => {
+    const ctx = new AuditContext('user-456', 'user');
+    const { sql, params } = withAuditColumns.update(
+      'subscriptions',
+      { status: 'active', amount: 199 },
+      'id = $1',
+      ['sub-123'],
+      ctx,
+    );
+
+    expect(sql).toContain('UPDATE subscriptions SET');
+    expect(sql).toContain('status =');
+    expect(sql).toContain('amount =');
+    expect(sql).toContain('updated_by =');
+    expect(sql).toContain('WHERE id =');
+    expect(params).toContain('user-456');
+    expect(params).toContain('sub-123');
+  });
+
+  it('handles multiple WHERE params', () => {
+    const ctx = AuditContext.system();
+    const { sql, params } = withAuditColumns.update(
+      'transactions',
+      { status: 'completed' },
+      'user_id = $1 AND id = $2',
+      ['u1', 't1'],
+      ctx,
+    );
+
+    expect(params).toEqual(['completed', 'system', 'u1', 't1']);
+    expect(sql).toContain('WHERE user_id =');
+    expect(sql).toContain('AND id =');
+  });
+});
+
+describe('AUDIT_TABLES', () => {
+  it('includes all core entity tables', () => {
+    expect(AUDIT_TABLES).toContain('subscriptions');
+    expect(AUDIT_TABLES).toContain('transactions');
+    expect(AUDIT_TABLES).toContain('users');
+    expect(AUDIT_TABLES).toContain('merchants');
+    expect(AUDIT_TABLES).toContain('loyalty');
+    expect(AUDIT_TABLES).toContain('payment_methods');
+    expect(AUDIT_TABLES).toContain('plans');
+  });
+});
diff --git a/backend/shared/db/auditColumns.ts b/backend/shared/db/auditColumns.ts
new file mode 100644
index 00000000..69a4f7e3
--- /dev/null
+++ b/backend/shared/db/auditColumns.ts
@@ -0,0 +1,138 @@
+/**
+ * Audit column utilities (issue #1801).
+ *
+ * Provides helpers for populating `created_by` and `updated_by` columns
+ * from the request context, and for building INSERT/UPDATE statements
+ * that include audit attribution.
+ *
+ * Usage:
+ *
+ *   import { AuditContext, withAuditColumns } from '../shared/db/auditColumns';
+ *
+ *   // In a request handler:
+ *   const ctx = AuditContext.fromRequest(req); // extracts actorId
+ *
+ *   // In a repository:
+ *   const { sql, params } = withAuditColumns.insert('subscriptions', {
+ *     user_id: userId, name, amount, ...
+ *   }, ctx);
+ *   await pool.query(sql, params);
+ */
+
+/** Tables that have created_by / updated_by columns. */
+export const AUDIT_TABLES = [
+  'subscriptions',
+  'transactions',
+  'users',
+  'merchants',
+  'loyalty',
+  'payment_methods',
+  'plans',
+] as const;
+
+export type AuditTable = (typeof AUDIT_TABLES)[number];
+
+/**
+ * Carries the actor identity through the request → repository path.
+ */
+export class AuditContext {
+  constructor(
+    public readonly actorId: string,
+    public readonly actorType: 'user' | 'system' | 'api_key' = 'user',
+  ) {}
+
+  /** Build from an HTTP request (extracts user ID from auth header / session). */
+  static fromRequest(req: {
+    userId?: string;
+    user?: { id?: string };
+    apiKey?: { id?: string };
+  }): AuditContext {
+    const actorId =
+      req.userId ??
+      req.user?.id ??
+      req.apiKey?.id ??
+      'system';
+    const actorType = req.apiKey?.id ? 'api_key' : actorId === 'system' ? 'system' : 'user';
+    return new AuditContext(actorId, actorType as 'user' | 'system' | 'api_key');
+  }
+
+  /** System context for background jobs / cron. */
+  static system(): AuditContext {
+    return new AuditContext('system', 'system');
+  }
+
+  /** Anonymous context for unauthenticated operations (e.g., signup). */
+  static anonymous(): AuditContext {
+    return new AuditContext('anonymous', 'system');
+  }
+}
+
+/**
+ * Build INSERT and UPDATE statements that include created_by / updated_by.
+ */
+export const withAuditColumns = {
+  /**
+   * Build an INSERT statement with created_by and updated_by.
+   *
+   * @param table target table
+   * @param data column → value map (without audit columns)
+   * @param ctx audit context providing the actor ID
+   * @returns { sql, params } ready for pool.query(sql, params)
+   */
+  insert(
+    table: string,
+    data: Record<string, unknown>,
+    ctx: AuditContext,
+  ): { sql: string; params: unknown[] } {
+    const columns = [...Object.keys(data), 'created_by', 'updated_by'];
+    const values = [...Object.values(data), ctx.actorId, ctx.actorId];
+    const placeholders = columns.map((_, i) => `$${i + 1}`).join(', ');
+
+    return {
+      sql: `INSERT INTO ${table} (${columns.join(', ')}) VALUES (${placeholders})`,
+      params: values,
+    };
+  },
+
+  /**
+   * Build an UPDATE statement that sets updated_by.
+   *
+   * @param table target table
+   * @param data column → value map (without audit columns)
+   * @param whereClause WHERE clause (e.g., "id = $1")
+   * @param whereParams params for the WHERE clause
+   * @param ctx audit context providing the actor ID
+   * @returns { sql, params } ready for pool.query(sql, params)
+   */
+  update(
+    table: string,
+    data: Record<string, unknown>,
+    whereClause: string,
+    whereParams: unknown[],
+    ctx: AuditContext,
+  ): { sql: string; params: unknown[] } {
+    const entries = Object.entries(data);
+    const setColumns = [...entries.map(([col]) => `${col} = $%`), 'updated_by = $%'];
+    const params: unknown[] = [];
+
+    let placeholderIdx = 1;
+    const setClause = setColumns
+      .map((s) => s.replace('$%', `$${placeholderIdx++}`))
+      .join(', ');
+
+    params.push(...entries.map(([, val]) => val));
+    params.push(ctx.actorId);
+
+    // Append WHERE params
+    const whereClauseWithPlaceholders = whereClause.replace(
+      /\$(\d+)/g,
+      (_, n) => `$${Number(n) + params.length - whereParams.length}`,
+    );
+    params.push(...whereParams);
+
+    return {
+      sql: `UPDATE ${table} SET ${setClause} WHERE ${whereClauseWithPlaceholders}`,
+      params,
+    };
+  },
+};
diff --git a/db/migrations/012_audit_columns_created_updated_by.sql b/db/migrations/012_audit_columns_created_updated_by.sql
new file mode 100644
index 00000000..ab0ce020
--- /dev/null
+++ b/db/migrations/012_audit_columns_created_updated_by.sql
@@ -0,0 +1,139 @@
+-- ── Migration 012: Audit columns — created/updated by (issue #1801) ──────────
+--
+-- Adds `created_by TEXT` and `updated_by TEXT` columns to core entity tables
+-- so every row records who created it and who last modified it.
+--
+-- All ALTERs are ADD COLUMN … NULL so they are instant on Postgres 11+
+-- (no table rewrite, no ACCESS EXCLUSIVE lock on data).
+--
+-- Run with:  psql $DATABASE_URL -f 012_audit_columns_created_updated_by.sql
+
+SET lock_timeout = '30s';
+
+-- ── Core entity tables ───────────────────────────────────────────────────────
+
+ALTER TABLE subscriptions ADD COLUMN IF NOT EXISTS created_by TEXT;
+ALTER TABLE subscriptions ADD COLUMN IF NOT EXISTS updated_by TEXT;
+
+ALTER TABLE transactions ADD COLUMN IF NOT EXISTS created_by TEXT;
+ALTER TABLE transactions ADD COLUMN IF NOT EXISTS updated_by TEXT;
+
+ALTER TABLE users        ADD COLUMN IF NOT EXISTS created_by TEXT;
+ALTER TABLE users        ADD COLUMN IF NOT EXISTS updated_by TEXT;
+
+ALTER TABLE merchants    ADD COLUMN IF NOT EXISTS created_by TEXT;
+ALTER TABLE merchants    ADD COLUMN IF NOT EXISTS updated_by TEXT;
+
+ALTER TABLE loyalty      ADD COLUMN IF NOT EXISTS created_by TEXT;
+ALTER TABLE loyalty      ADD COLUMN IF NOT EXISTS updated_by TEXT;
+
+ALTER TABLE payment_methods ADD COLUMN IF NOT EXISTS created_by TEXT;
+ALTER TABLE payment_methods ADD COLUMN IF NOT EXISTS updated_by TEXT;
+
+ALTER TABLE plans        ADD COLUMN IF NOT EXISTS created_by TEXT;
+ALTER TABLE plans        ADD COLUMN IF NOT EXISTS updated_by TEXT;
+
+-- ── Indexes for querying by creator/updater ──────────────────────────────────
+
+CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_subscriptions_created_by
+  ON subscriptions (created_by) WHERE created_by IS NOT NULL;
+CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_subscriptions_updated_by
+  ON subscriptions (updated_by) WHERE updated_by IS NOT NULL;
+
+CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_transactions_created_by
+  ON transactions (created_by) WHERE created_by IS NOT NULL;
+CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_transactions_updated_by
+  ON transactions (updated_by) WHERE updated_by IS NOT NULL;
+
+CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_users_created_by
+  ON users (created_by) WHERE created_by IS NOT NULL;
+
+CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_merchants_created_by
+  ON merchants (created_by) WHERE created_by IS NOT NULL;
+
+-- ── Trigger function: auto-update updated_at and updated_by ─────────────────
+-- This trigger fires on UPDATE and sets updated_at = now() automatically.
+-- updated_by must be set by the application; the trigger only updates
+-- updated_at so the application doesn't have to manage timestamps manually.
+
+CREATE OR REPLACE FUNCTION set_updated_at()
+RETURNS TRIGGER AS $$
+BEGIN
+  NEW.updated_at = now();
+  RETURN NEW;
+END;
+$$ LANGUAGE plpgsql;
+
+DROP TRIGGER IF EXISTS trg_subscriptions_updated_at ON subscriptions;
+CREATE TRIGGER trg_subscriptions_updated_at
+  BEFORE UPDATE ON subscriptions
+  FOR EACH ROW EXECUTE FUNCTION set_updated_at();
+
+DROP TRIGGER IF EXISTS trg_transactions_updated_at ON transactions;
+CREATE TRIGGER trg_transactions_updated_at
+  BEFORE UPDATE ON transactions
+  FOR EACH ROW EXECUTE FUNCTION set_updated_at();
+
+DROP TRIGGER IF EXISTS trg_users_updated_at ON users;
+CREATE TRIGGER trg_users_updated_at
+  BEFORE UPDATE ON users
+  FOR EACH ROW EXECUTE FUNCTION set_updated_at();
+
+DROP TRIGGER IF EXISTS trg_merchants_updated_at ON merchants;
+CREATE TRIGGER trg_merchants_updated_at
+  BEFORE UPDATE ON merchants
+  FOR EACH ROW EXECUTE FUNCTION set_updated_at();
+
+DROP TRIGGER IF EXISTS trg_loyalty_updated_at ON loyalty;
+CREATE TRIGGER trg_loyalty_updated_at
+  BEFORE UPDATE ON loyalty
+  FOR EACH ROW EXECUTE FUNCTION set_updated_at();
+
+DROP TRIGGER IF EXISTS trg_payment_methods_updated_at ON payment_methods;
+CREATE TRIGGER trg_payment_methods_updated_at
+  BEFORE UPDATE ON payment_methods
+  FOR EACH ROW EXECUTE FUNCTION set_updated_at();
+
+DROP TRIGGER IF EXISTS trg_plans_updated_at ON plans;
+CREATE TRIGGER trg_plans_updated_at
+  BEFORE UPDATE ON plans
+  FOR EACH ROW EXECUTE FUNCTION set_updated_at();
+
+-- ── Analyse ──────────────────────────────────────────────────────────────────
+ANALYZE subscriptions;
+ANALYZE transactions;
+ANALYZE users;
+ANALYZE merchants;
+ANALYZE loyalty;
+ANALYZE payment_methods;
+ANALYZE plans;
+
+-- @down
+DROP TRIGGER IF EXISTS trg_plans_updated_at ON plans;
+DROP TRIGGER IF EXISTS trg_payment_methods_updated_at ON payment_methods;
+DROP TRIGGER IF EXISTS trg_loyalty_updated_at ON loyalty;
+DROP TRIGGER IF EXISTS trg_merchants_updated_at ON merchants;
+DROP TRIGGER IF EXISTS trg_users_updated_at ON users;
+DROP TRIGGER IF EXISTS trg_transactions_updated_at ON transactions;
+DROP TRIGGER IF EXISTS trg_subscriptions_updated_at ON subscriptions;
+DROP FUNCTION IF EXISTS set_updated_at();
+DROP INDEX CONCURRENTLY IF EXISTS idx_merchants_created_by;
+DROP INDEX CONCURRENTLY IF EXISTS idx_users_created_by;
+DROP INDEX CONCURRENTLY IF EXISTS idx_transactions_updated_by;
+DROP INDEX CONCURRENTLY IF EXISTS idx_transactions_created_by;
+DROP INDEX CONCURRENTLY IF EXISTS idx_subscriptions_updated_by;
+DROP INDEX CONCURRENTLY IF EXISTS idx_subscriptions_created_by;
+ALTER TABLE plans        DROP COLUMN IF EXISTS updated_by;
+ALTER TABLE plans        DROP COLUMN IF EXISTS created_by;
+ALTER TABLE payment_methods DROP COLUMN IF EXISTS updated_by;
+ALTER TABLE payment_methods DROP COLUMN IF EXISTS created_by;
+ALTER TABLE loyalty      DROP COLUMN IF EXISTS updated_by;
+ALTER TABLE loyalty      DROP COLUMN IF EXISTS created_by;
+ALTER TABLE merchants    DROP COLUMN IF EXISTS updated_by;
+ALTER TABLE merchants    DROP COLUMN IF EXISTS created_by;
+ALTER TABLE users        DROP COLUMN IF EXISTS updated_by;
+ALTER TABLE users        DROP COLUMN IF EXISTS created_by;
+ALTER TABLE transactions DROP COLUMN IF EXISTS updated_by;
+ALTER TABLE transactions DROP COLUMN IF EXISTS created_by;
+ALTER TABLE subscriptions DROP COLUMN IF EXISTS updated_by;
+ALTER TABLE subscriptions DROP COLUMN IF EXISTS created_by;