feat: idempotent refunds - #1900

• Closes Add idempotent postage refunds with atomic conflict handling #1546

Summary

Resolves issue by introducing durable idempotency support and atomic conflict handling for the postage refund endpoint (POST /api/v1/postage/:messageId/refund).

Problem

Refund and settlement requests can race or be retried after client timeouts or network disruptions. Without idempotency and atomic state transitions, retries could result in duplicate side-effects or ambiguous escrow states.

Changes Implemented

1. Refund Route Idempotency (src/routes/api/v1/postage/$messageId/refund.ts):

   • Integrated optional X-Idempotency-Key header handling.
   • Uses acquireIdempotency to lease actor-scoped idempotency keys (scoped per recipient).
   • Replays stored successful responses (200 OK) and terminal-state error responses (409 Conflict) with header x-idempotency-replayed: true.
   • Handles in_progress concurrency locks with a 409 Conflict error.
   • Records successful refunds and terminal state conflict outcomes via recordIdempotency.
   • Added comprehensive JSDoc documentation and usage examples.

2. Unit Tests (tests/unit/api/postage-refund-idempotency.test.ts):

   • Added 14 unit tests covering:
     • Deterministic terminal state errors (409 Conflict) when refunding already-refunded or already-settled postage.
     • Concurrency races ensuring only one refund side-effect occurs when multiple calls race without idempotency keys.
     • Concurrency races between settlement and refund operations (ensuring only one wins).
     • Refund losing cleanly after settlement has completed.
     • Success and terminal error replay via checkIdempotency and recordIdempotency.
     • Actor isolation ensuring different recipients cannot replay each other's idempotency records.
     • Network retry scenarios and data integrity preservation.

3. Documentation:

   • Created docs/api/REFUND_IDEMPOTENCY.md with detailed API specifications, flow diagrams, error payload contracts, security/actor isolation rationale, and client implementation guidelines.
   • Updated docs/api/README.md and src/routes/api/v1/postage/README.md to reference REFUND_IDEMPOTENCY.md and the refund idempotency contract.

Acceptance Criteria Verification

• Only one refund side effect occurs under concurrency.
• Refund cannot win after settlement.
• Retry after refund returns a deterministic replayed response (200 OK or 409 Conflict).
• Concurrency tests cover settlement-versus-refund.

Fixes #1546