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>

monitoring/prometheus.yml

@@ -45,6 +45,13 @@ scrape_configs:
         labels:
           service: "prometheus"
 
+  # Redis Exporter — Redis memory, connections, command stats
+  - job_name: "redis"
+    static_configs:
+      - targets: ["redis-exporter:9121"]
+        labels:
+          service: "redis"
+
   # Alertmanager
   - job_name: "alertmanager"
     static_configs: