feat: CDN edge caching, DB query monitoring, ETag middleware, job priority management

[bulusmalaga]

closes #762
closes #763
closes #764
closes #765

feat: CDN edge caching, DB query monitoring, ETag middleware, job priority management

a. CDN edge caching

• app.json: add extra.cdn config (origin shield, TTLs, regions, warm paths)
• metro.config.js: explicit assetExts for CDN content-hash filenames
• scripts/cdn-cache-warm.sh: post-deploy surrogate-key purge + cache warming
• scripts/cdn-regional-monitor.js: Fastly real-time per-region hit-rate monitoring
• docs/cdn-edge-caching.md: full CDN architecture documentation

b. Database query performance monitoring

• backend/shared/query/queryPerformanceMonitor.ts: extends SlowQueryMonitor with
  EXPLAIN ANALYZE integration, seq/index scan tracking, index recommendation
  engine (high/medium/low severity), query plan analysis, alert ring-buffer,
  Prometheus metrics, and dashboard snapshot
• backend/server.ts: add GET /metrics/db-performance endpoint
• docs/database-performance.md: query logging, slow detection, index stats docs

c. ETag caching middleware

• backend/shared/middleware/etagMiddleware.ts: SHA-256 strong ETags, If-None-Match
  -> 304 Not Modified, per-path Cache-Control (plans 5m, features 10m, public 1h,
  no-store for graphql/health/metrics), stale-while-revalidate=60, auth bypass
• backend/shared/middleware/index.ts: export new ETag symbols
• backend/server.ts: wire applyETagToRawHandler into HTTP server
• docs/etag-caching.md: ETag generation, 304 flow, TTL table, auth bypass docs

d. Job priority management

• backend/shared/queue/retryPolicy.ts: exponential backoff with full-jitter,
  per-priority max attempts (critical=10, high=7, normal=5, low=3)
• backend/shared/queue/deadLetterQueue.ts: in-memory ring-buffer + BullMQ
  persistence, 7-day retention, replay support, onDeadLetter callback
• backend/shared/queue/jobRateLimiter.ts: sliding-window token bucket per service,
  pre-configured defaults for Stripe/SendGrid/Twilio/Expo/blockchain-rpc
• backend/shared/queue/jobScheduler.ts: 6-field cron scheduler on WeightedFairQueue,
  second-boundary debounce, skipIfRunning guard, manual fireNow() for testing
• backend/shared/queue/jobMonitoringDashboard.ts: unified snapshot from WFQ + DLQ
  • rate limiter + cron scheduler, health states, Prometheus metrics export
• backend/shared/queue/index.ts: export all new queue symbols
• docs/job-queue.md: priority table, retry formula, DLQ replay, cron docs

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