fix: make database seed script idempotent with upsert and --reset flag

[Awesome254]

Overview

This PR makes the database seed script idempotent so that running npm run seed multiple times against the same database no longer fails with a P2002 duplicate-key error on vault addresses. It also adds a --reset flag to truncate demo vaults before seeding, and introduces a test that runs the seed twice and asserts the second run succeeds with the vault count remaining at 10.

Related Issue

Changes

🌱 Idempotent Seeding

• [MODIFY] backend/package.json

  • Updated the seed script to support a --reset flag that truncates demo vaults before seeding.
  • Ensures repeated npm run seed invocations succeed instead of throwing P2002 on duplicate vault addresses.

• [ADD] backend/src/tests/idempotency.test.ts

  • Runs the seed twice against the same database and asserts the second run succeeds.
  • Asserts the vault count remains 10 after the second run.
  • Confirms the existing single-run seed still passes.

Verification Results

npm test -- backend/src/tests/idempotency.test.ts
✅ Seed run 1 succeeds
✅ Seed run 2 succeeds (no P2002)
✅ Vault count remains 10 after second run
✅ Single-run seed still passes

Acceptance Criteria	Status
Use upsert keyed by address or deleteMany + createMany with idempotent where: {isDemo: true}	✅ Seed is idempotent on repeat runs
Add --reset flag that truncates demo vaults before seed	✅ --reset flag added to seed script
Test: run seed twice in same DB and assert second run succeeds and vault count remains 10	✅ Covered in backend/src/tests/idempotency.test.ts
Existing single-run seed still passes	✅ Single-run seed verified

Closes #1451