Add fake payment lifecycle API

[SimoneMariaRomeo]

Summary

• adds a Winterspec-native fake payment API for sending, listing, fetching, completing, and cancelling payments
• stores fake payments in the existing in-memory DB with status timestamps and bounty metadata
• makes send retry-safe when callers provide an idempotency key

Verification

• bun x biome check lib\db\schema.ts lib\db\db-client.ts lib\payments\schemas.ts routes\payments tests\routes\payments
• bun test
• bun run build
• bun x tsc --noEmit
• git diff --check

/claim #1

[SimoneMariaRomeo]

Verification is complete locally: bun test, build, type-check, Biome check on touched paths, and git diff --check all pass. The fake payment lifecycle API is ready for review/reward when convenient.