Add fake payment API

[Vinzz2303]

/claim #1

Summary:

• add a Winterspec fake payments API with send/list/get/complete/cancel/fail routes
• persist in-memory payment records with recipient, amount, currency, bounty URL, status, timestamps, and optional idempotency key
• make /payments/send retry-safe by replaying the original payment for duplicate idempotency keys
• add status transition guards so final payments are not mutated again
• add route tests for send/list, idempotent replay, and completion guard behavior
• document the payment endpoints in the README

Verification:

• npx.cmd --yes bun test tests/routes/payments.test.ts
• npx.cmd --yes @biomejs/[email protected] check lib\db\schema.ts lib\db\db-client.ts routes\payments tests\routes\payments.test.ts README.md
• git diff --check

[Vinzz2303]

Validation update for PR #36:

npx.cmd --yes bun test tests/routes/payments.test.ts
# 3 pass, 0 fail

npx.cmd --yes @biomejs/biome@1.8.3 check lib\db\schema.ts lib\db\db-client.ts routes\payments tests\routes\payments.test.ts README.md
# Checked 10 files. No fixes applied.

git diff --check
# no output

This confirms the focused payment route tests, Biome check, and whitespace check pass on the current PR branch.

[Vinzz2303]

Friendly follow-up: validation for this PR is attached above (�un test, Biome check, and git diff --check all passed). This stays narrowly scoped to the fake payment API/payment route slice for issue #1. Happy to adjust anything if there is a preferred review direction.