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/src/admin/admin.module.ts

@@ -1,10 +1,11 @@
 import { Module } from '@nestjs/common';
 import { AdminController } from './admin.controller.js';
 import { AdminHealthController } from './admin-health.controller.js';
+import { AdminJobsController } from './admin-jobs.controller.js';
 import { AdminGuard } from './admin.guard.js';
 
 @Module({
-  controllers: [AdminController, AdminHealthController],
+  controllers: [AdminController, AdminHealthController, AdminJobsController],
   providers: [AdminGuard],
 })
 export class AdminModule {}