feat(monitoring): add Redis exporter + Sentry docs to deployment guide

- Add redis-exporter container to docker-compose (oliver006/redis_exporter, 32MB)
- Add Redis scrape target to Prometheus config
- Add 4 Redis alert rules: RedisDown, HighMemory, HighConnections, RejectedConnections
- Document Step 19b (Sentry Error Tracking) in Hetzner deployment guide
- Document Step 19c (Redis Monitoring) in Hetzner deployment guide
- Update resource budget and port reference tables

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/proposals/decouple-modules.md

@@ -1,7 +1,7 @@
   Analytics Dependency Status: ❌ NOT FIXED                                                                                                                                                                                                                                                                                        ─
-                  
-  The MetricsProvider pattern exists in contracts/metrics.py, but admin_stores.py still has hard imports:                                                                                                                                                                                                                         
-                  
+
+  The MetricsProvider pattern exists in contracts/metrics.py, but admin_stores.py still has hard imports:
+
   File: app/modules/tenancy/routes/api/admin_stores.py
   ┌──────┬────────────────────────────────────────────────────────────────────────┬─────────────────────────────────────────────┐
   │ Line │                                 Import                                 │                   Used In                   │
@@ -914,4 +914,4 @@ Modular Architecture Analysis
  2. Each module owns its metrics - no cross-module coupling
  3. Optional modules truly optional - can be removed without breaking app
  4. Easy to add new metrics - just implement protocol in your module
- 5. Both dashboards supported - store (per-store) and admin (platform-wide)
+ 5. Both dashboards supported - store (per-store) and admin (platform-wide)