Add fake payment lifecycle API

[tarunag10]

Summary

• add in-memory fake payment records with send/list/get/complete/cancel lifecycle routes
• support retry-safe sends with either an idempotency_key body field or Idempotency-Key header
• preserve default middleware payment state across requests with a shared in-memory DB
• document the payment endpoints in the README

/claim #1

Tests

• npx bun test tests/routes/payments-default-middleware.test.ts tests/routes/payments.test.ts
• npx bun test
• npx bun run build
• npx tsc --noEmit
• npx biome check README.md lib/db/schema.ts lib/db/db-client.ts lib/middleware/with-db.ts lib/payments/route-schemas.ts routes/payments tests/routes/payments.test.ts tests/routes/payments-default-middleware.test.ts
• git diff --check

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Scope default DB instance per app context

Move DB initialization out of module scope: creating defaultDb once at import time makes all routes that rely on withDb share state across the entire process, so separate Winterspec bundles or test runs in the same process can see each other’s payments/things and become order-dependent. This introduces cross-instance data bleed rather than just per-request persistence.

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