**Title:** feat(rate-limit): cap per-provider queue depth with reject-on-overflow

[SHEROSE0]

#closes #409
Description:
This PR implements a configurable queue depth cap to prevent unbounded memory accumulation when upstream providers are persistently slower than the refill rate.

Changes:

• Queue Capping: Integrated maxQueueDepth validation in src/rateLimit.ts. Requests exceeding this limit now immediately reject with RATE_LIMIT_QUEUE_CAP_EXCEEDED instead of queueing indefinitely.
• Metrics: Integrated with src/webhookMetrics.ts to track rejections for better observability.
• Backpressure Policy: Defined deterministic behavior: requests rejected under load can now be explicitly routed to a Dead Letter Queue (DLQ) by the caller.
• Testing: Added src/rateLimit.test.ts using fake timers to verify FIFO processing and hard rejection limits.

Security & Stability:

• DoS Protection: Mitigates memory-exhaustion (OOM) scenarios where thousands of closures are captured in an ever-growing queue.
• Fail-Fast Semantics: Allows upstream services to detect backpressure immediately.

Acceptance Criteria Checklist:

• Configurable maxQueueDepth applied per provider.
• Typed error rejection on overflow.
• Metrics recorded for QUEUE_CAP_EXCEEDED.
• FIFO order preserved for queued items.
• Test coverage ≥ 95%.
  #closes