feat: monitoring exporters, alerting, rate limiting, backups, incident runbook

- Add 4 Prometheus exporters: nginx (9113), redis (9121), postgres (9187), node (9100)
- Create 8 alerting rules in monitoring/prometheus/alerts.yml
- Update prometheus.yml with all scrape targets + rule_files
- WS connection rate limiting: max 5/IP/60s (Redis INCR)
- Admin login rate limiting: max 5/IP/5min (Redis INCR)
- 3 backup scripts: daily to R2 (14d), weekly full (8w), verify with restore test
- docs/incident-runbook.md with 5-tier triage + escalation
- Update CD workflow: full prometheus config + alerts seeding

monitoring/prometheus/prometheus.yml

@@ -1,10 +1,28 @@
 global:
   scrape_interval: 15s
 
+rule_files:
+  - "/etc/prometheus/alerts.yml"
+
 scrape_configs:
   - job_name: "chat"
     static_configs:
       - targets: ["chat:3000"]
+  - job_name: "playout"
+    static_configs:
+      - targets: ["playout:8081"]
+  - job_name: "nginx"
+    static_configs:
+      - targets: ["nginx-exporter:9113"]
+  - job_name: "redis"
+    static_configs:
+      - targets: ["redis-exporter:9121"]
+  - job_name: "postgres"
+    static_configs:
+      - targets: ["postgres-exporter:9187"]
+  - job_name: "node"
+    static_configs:
+      - targets: ["node-exporter:9100"]
   - job_name: "prometheus"
     static_configs:
       - targets: ["prometheus:9090"]