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>

docker-compose.yml

@@ -229,6 +229,28 @@ services:
     networks:
       - monitoring
 
+  redis-exporter:
+    image: oliver006/redis_exporter:latest
+    restart: always
+    profiles:
+      - full
+    ports:
+      - "127.0.0.1:9121:9121"
+    environment:
+      REDIS_ADDR: redis://redis:6379
+    depends_on:
+      redis:
+        condition: service_healthy
+    mem_limit: 32m
+    healthcheck:
+      test: ["CMD-SHELL", "wget -qO- http://localhost:9121/health || exit 1"]
+      interval: 30s
+      timeout: 10s
+      retries: 3
+    networks:
+      - backend
+      - monitoring
+
   alertmanager:
     image: prom/alertmanager:latest
     restart: always