[#540] Add concurrency-safe idempotency for charges

[arisu6804]

Summary

Adds a production-shaped charge write path with concurrency-safe, tenant-scoped idempotency for POST /api/v1/charges.

What changed

• Added a typed ChargeIdempotencyStore contract and atomic in-memory claim state machine.
• Added canonical fingerprints over method, path, tenant, and charge payload.
• Added 24-hour replay retention, cloned response replay, conflict detection, and claim release on failed execution.
• Added tenant-isolated charge storage and a tenant-filtered list endpoint.
• Added validation for safe idempotency keys, positive safe amounts, currency, source, and description limits.
• Added OpenAPI route registration and contributor documentation.

Acceptance criteria

• Same key and same body replays the saved status/body without creating a second charge (src/routes/charges.ts, route test).
• Same key with a different body returns 409 idempotency_conflict.
• Concurrent claim returns 409 request_in_progress before a second side effect.
• Key reuse after the 24-hour TTL is treated as a new request.
• Missing keys preserve normal at-least-once behavior.
• Keys are scoped by tenant and cannot collide across tenants.
• Invalid or oversized keys return a structured 400 response.

Verification

• npm run build
• node --test dist/charges.test.js — 17/17 passing
• npm run lint

The repository-wide legacy test command was also attempted. Existing parallel test-isolation suites report unrelated shared-state/timeouts in this checkout; the new focused suite is deterministic and passes independently.

Operational note

The shipped adapter is intentionally in-memory, as requested by the issue. A multi-process deployment must provide a shared implementation of ChargeIdempotencyStore before relying on exactly-once behavior across process boundaries.

Closes #540