Migrate Float monetary fields to Decimal in Prisma schema #237

Problem Statement. Money is stored as Float in Campaign.budget, Claim.amount,
AidPackage.totalAmount/claimedAmount/remainingAmount, and BalanceLedger.amount. Doubles
cannot represent one USDC cent exactly and SQLite further mangles them.

Why it matters. Floats in money-handling systems are a class of bug. The schema is
the cross-language boundary; a Swift or Go client that reads these values can pick up
0.1 + 0.2 == 0.30000000000000004 and proceed to disburse incorrectly.

Technical Context. prisma/schema.prisma. Decimal is supported by Prisma + Postgres
natively; for SQLite the round-trip goes via string.

Expected Outcome. All monetary fields use Decimal with String representation for
the JS client. Existing data is migrated in a single script using BigInt arithmetic.

Acceptance Criteria.

• New schema field types: Decimal @db.Decimal(38, 18).
• A migration script prisma/migrations/<ts>_floats_to_decimal/migration.sql covers the
  four models.
• A unit-test in app/backend/test/money-roundtrip.spec.ts proves
  Decimal("1000.5") round-trips losslessly through Python, JS, and Postgres client.
• API responses explicitly serialize decimals as strings (use a custom DecimalSerializerInterceptor).

Implementation Notes. Avoid parseFloat anywhere in the consumer code; use Decimal js.

Files or modules likely to be affected. prisma/schema.prisma,
prisma/migrations/*, src/common/decimal.ts (new), src/main.ts.

Dependencies. #30 (data migration tooling).

Difficulty. Hard
Estimated effort. L

Backlog item #29 from `docs/maintainer-issue-backlog.md.