feat: replace floating-point monetary storage with exact BigInt stroops

[Muhammadjazuli]

Summary

Closes #123

Replaces all binary floating-point monetary fields with exact-integer stroop values (1 XLM = 10,000,000 stroops) across the Prisma schema, service layer, controller, API types, and tests. This eliminates silent IEEE-754 precision loss in reward calculations and makes asset identity explicit on every monetary record.

---

Changes

Prisma schema (prisma/schema.prisma)

Table	Old field	New field
Module	reward Float	rewardStroops BigInt + assetCode, assetIssuer, assetNetwork
Transaction	amount Float	amountStroops BigInt + assetCode, assetIssuer, assetNetwork
Referral	bonusAmount Float?	bonusAmountStroops BigInt?

Migration (prisma/migrations/20260821000000_exact_monetary_storage/migration.sql)

• Converts legacy float values using ROUND(old * 10_000_000)::BIGINT (nearest stroop)
• Adds CHECK (col >= 0) constraints on all stroop columns
• Wrapped in BEGIN / COMMIT — atomic
• Rollback notes documented in SQL comments

New utility module (src/utils/money.ts)

• xlmToStroops, stroopsToXlmString, xlmStringToStroops
• legacyFloatXlmToStroops — explicit rounding, throws on NaN / Infinity / negative
• addStroops, subtractStroops (throws on underflow), multiplyStroops (rational BigInt), clampStroops
• formatStroops, assertInRange, isValidStroopAmount, MoneyError

Service (src/services/reward.service.ts)

• All internal amounts use bigint stroops throughout
• Difficulty multipliers expressed as [numerator, denominator] rational pairs — no floats
• Stellar SDK sendPayment receives 7-decimal XLM strings via stroopsToXlmString

Controller (src/controllers/reward.controller.ts)

• Parses incoming amounts via xlmStringToStroops (rejects > 7 decimal places)
• Serialises bigint → XLM string at every API response boundary

Types (src/types/reward.types.ts)

• Transaction.amount / Balance fields changed from number to string (7-decimal XLM)

---

Tests

File	Tests	What is covered
tests/unit/money.test.ts	83	Conversion, rounding, overflow, boundary, arithmetic, integration
tests/unit/reward.service.test.ts	48	BigInt stroop arithmetic, Stellar receives XLM strings not numbers
tests/unit/reward.controller.test.ts	22	Mock data updated to BigInt/XLM string; all 22 pass

---

Verification

pnpm lint          → clean (0 errors, 0 warnings)
pnpm test:coverage → 741 passed | 3 skipped (DB-dependent, no Postgres in CI)

The 3 skipped tests are DB integration tests that have always been skipped in the test environment without a running Postgres instance — unrelated to this change.