Add fake payment API

[andresreibel]

Summary

• add fake payment records to the in-memory database schema
• add payment send/list/get/complete/cancel/fail Winterspec routes
• support idempotency keys so retries do not create duplicate fake transfers
• support bounty/repository filters: recipient, status, owner, repo, repository, bounty id, and issue number
• return null for missing payment lookups/status updates instead of leaking undefined
• document the fake payment API with request examples
• add route coverage for send, idempotent retry, list filters, get, complete, cancel, fail, and missing-payment flows

/claim #1

Tests

• bunx biome check lib/db/db-client.ts lib/db/schema.ts routes/payments/send.ts routes/payments/list.ts routes/payments/get.ts routes/payments/complete.ts routes/payments/cancel.ts routes/payments/fail.ts tests/routes/payments.test.ts README.md
• bun test
• bun run build
• git diff --check