feat(infra): add alerting, network segmentation, and ops docs (Steps 19-24)

- Prometheus alert rules (host, container, API, Celery, target-down)
- Alertmanager with email routing (critical 1h, warning 4h repeat)
- Docker network segmentation (frontend/backend/monitoring)
- Incident response runbook with 8 copy-paste runbooks
- Environment variables reference (55+ vars documented)
- Hetzner setup docs updated with Steps 19-24
- Launch readiness updated with Feb 2026 infrastructure status

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

monitoring/prometheus.yml

@@ -5,6 +5,16 @@ global:
   scrape_interval: 15s
   evaluation_interval: 15s
 
+# ─── Alerting ────────────────────────────────────────────────────────────
+alerting:
+  alertmanagers:
+    - static_configs:
+        - targets: ["alertmanager:9093"]
+
+rule_files:
+  - /etc/prometheus/alert.rules.yml
+
+# ─── Scrape Configs ─────────────────────────────────────────────────────
 scrape_configs:
   # Orion API — /metrics endpoint (prometheus_client)
   - job_name: "orion-api"
@@ -34,3 +44,10 @@ scrape_configs:
       - targets: ["localhost:9090"]
         labels:
           service: "prometheus"
+
+  # Alertmanager
+  - job_name: "alertmanager"
+    static_configs:
+      - targets: ["alertmanager:9093"]
+        labels:
+          service: "alertmanager"