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:

monitoring/prometheus/alert.rules.yml

@@ -125,6 +125,47 @@ groups:
           summary: "Celery queue backlog exceeding 100 tasks"
           description: "Queue {{ $labels.queue }} has {{ $value | printf \"%.0f\" }} pending tasks for 10 minutes."
 
+  # =========================================================================
+  # REDIS ALERTS (redis-exporter)
+  # =========================================================================
+  - name: redis
+    rules:
+      - alert: RedisDown
+        expr: redis_up == 0
+        for: 1m
+        labels:
+          severity: critical
+        annotations:
+          summary: "Redis is down"
+          description: "Redis exporter cannot connect to Redis for 1 minute. Background tasks (emails, Celery) are not processing."
+
+      - alert: RedisHighMemoryUsage
+        expr: redis_memory_used_bytes / redis_memory_max_bytes * 100 > 80
+        for: 5m
+        labels:
+          severity: warning
+        annotations:
+          summary: "Redis memory usage above 80%"
+          description: "Redis is using {{ $value | printf \"%.1f\" }}% of its max memory. Consider increasing mem_limit or investigating queue backlog."
+
+      - alert: RedisHighConnectionCount
+        expr: redis_connected_clients > 50
+        for: 5m
+        labels:
+          severity: warning
+        annotations:
+          summary: "Redis has {{ $value }} connected clients"
+          description: "Unusually high number of Redis connections. May indicate connection leaks."
+
+      - alert: RedisRejectedConnections
+        expr: increase(redis_rejected_connections_total[5m]) > 0
+        for: 0m
+        labels:
+          severity: critical
+        annotations:
+          summary: "Redis is rejecting connections"
+          description: "Redis rejected {{ $value | printf \"%.0f\" }} connections in the last 5 minutes. Clients cannot connect."
+
   # =========================================================================
   # PROMETHEUS SELF-MONITORING
   # =========================================================================