feat: implement backpressure handling for queue processing

[Nimatstar]

Summary

Implements a backpressure mechanism for queue processing that monitors queue depth and active job counts, applies rate-limiting delays when thresholds are breached, and exposes monitoring snapshots via REST endpoints.

Changes

• src/queues/backpressure/backpressure.service.ts - BackpressureService:
  • evaluate(queueName, depth, active) - resolves
    ormal / warning / critical level using existing QUEUE_HEALTH_THRESHOLDS constants; returns { level, rateLimitActive, delayMs }
  • Rate-limiting delays: warning ? 1 s, critical ? 5 s
  • Deduped ERROR log on first critical breach; LOG on recovery
  • getSnapshot(queue) / getAllSnapshots() - monitoring data per queue
• src/queues/backpressure/backpressure.controller.ts - GET /queues/backpressure (all) and GET /queues/backpressure/:queue (single)
• src/queues/backpressure/backpressure.module.ts - module wiring, exports service

Usage

s // In a queue producer before enqueuing: const state = this.backpressureService.evaluate(queueName, depth, active); if (state.rateLimitActive) { if (state.level === 'critical') throw new TooManyRequestsException(); await sleep(state.delayMs); }

Acceptance Criteria

• Backpressure mechanism implemented (evaluate() with level resolution)
• Rate limiting on queue processing (delayMs +
  ateLimitActive flag)
• Queue depth monitoring (getSnapshot / getAllSnapshots endpoints)
• Alerting on high queue depth (ERROR/WARN logs on threshold breach)

closes #608

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