Add fake payment API lifecycle

[J3d1-fm]

Adds a Winterspec-native fake payment API for issue #1.

Implemented endpoints:

• POST /payments/send
• GET /payments/list
• GET /payments/get
• POST /payments/complete
• POST /payments/cancel

The send endpoint supports optional idempotency keys so retries return the original fake payment instead of creating duplicates. Payment records are persisted in the existing in-memory zod/zustand DB and can be filtered by status, recipient, or repository.

Verification:

• /private/tmp/bun-runtime/bun-darwin-aarch64/bun test
• /private/tmp/bun-runtime/bun-darwin-aarch64/bun x biome check lib/db/schema.ts lib/db/db-client.ts routes/payments/send.ts routes/payments/list.ts routes/payments/get.ts routes/payments/complete.ts routes/payments/cancel.ts tests/routes/payments/payments.test.ts
• /private/tmp/bun-runtime/bun-darwin-aarch64/bun run build

/claim #1