feat(monitoring): add alerting stack and maintenance scripts

Medium priority stability improvements:

Alerting:
- Add vmalert for evaluating Prometheus alert rules
- Add alertmanager for alert routing and grouping
- Add alert-notifier service for Telegram/ntfy notifications
- Enable cadvisor scraping in prometheus config

Disk Monitoring:
- Add check-disk-space.sh for hourly disk monitoring
- Alert on 80% (warning) and 90% (critical) thresholds
- Auto-cleanup Docker when disk is critical
- Add com.manacore.disk-check.plist for LaunchD

Weekly Reports:
- Add weekly-report.sh for system health summary
- Includes: backup status, disk usage, container health,
  database stats, error log summary
- Runs every Sunday at 10 AM via LaunchD

Health Check Updates:
- Add checks for vmalert, alertmanager, alert-notifier

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docker/prometheus/prometheus.yml

@@ -9,11 +9,11 @@ global:
 rule_files:
   - /etc/prometheus/alerts.yml
 
-# Alertmanager configuration (optional, for future use)
-# alerting:
-#   alertmanagers:
-#     - static_configs:
-#         - targets: []
+# Alertmanager configuration
+alerting:
+  alertmanagers:
+    - static_configs:
+        - targets: ['alertmanager:9093']
 
 scrape_configs:
   # Prometheus self-monitoring
@@ -30,10 +30,10 @@ scrape_configs:
         target_label: instance
         replacement: 'mac-mini'
 
-  # Docker container metrics via cAdvisor (disabled - container not deployed)
-  # - job_name: 'cadvisor'
-  #   static_configs:
-  #     - targets: ['cadvisor:8080']
+  # Docker container metrics via cAdvisor
+  - job_name: 'cadvisor'
+    static_configs:
+      - targets: ['cadvisor:8080']
 
   # PostgreSQL metrics
   - job_name: 'postgres'