feat(M5-008,M6-001-005): session hardening tests + BullMQ job queue foundation

- M5-008: Add session-hardening.test.ts verifying /model switch, /agent switch,
  session resume with conversationHistory, and metrics tracking (tokens + message count)
- M6-001: Install bullmq, configure connection via VALKEY_URL (redis://localhost:6380)
- M6-002: Create QueueService (apps/gateway/src/queue/) with typed job definitions
  (SummarizationJob, GCJob, TierManagementJob), worker registration with exponential
  backoff, queue health status, and admin API helpers (listJobs, retryJob, pause/resume)
- M6-003: Migrate summarization cron → BullMQ repeatable job (SUMMARIZATION_CRON env)
- M6-004: Migrate session GC cron → BullMQ repeatable job (SESSION_GC_CRON env)
- M6-005: Migrate tier management cron → BullMQ repeatable job (TIER_MANAGEMENT_CRON env)
- Add AdminJobsController for queue/job management at /api/admin/jobs
- All three quality gates pass: typecheck, lint, format:check
- 319 tests pass

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/gateway/package.json

@@ -42,6 +42,7 @@
     "@opentelemetry/semantic-conventions": "^1.40.0",
     "@sinclair/typebox": "^0.34.48",
     "better-auth": "^1.5.5",
+    "bullmq": "^5.71.0",
     "class-transformer": "^0.5.1",
     "class-validator": "^0.15.1",
     "dotenv": "^17.3.1",