Add Uptime Kuma monitoring integration with dedicated hooks

Implement Uptime Kuma Push Monitor support with separate hook files
following separation of concerns principle.

Changes:
- Add UPTIME_KUMA_PUSH_URL to .env.example
- Create dedicated uptime-kuma-success.sh hook
- Create dedicated uptime-kuma-error.sh hook
- Update config.yaml to call both ntfy and Uptime Kuma hooks
- Each notification service has its own file for modularity

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

config.yaml

@@ -80,13 +80,15 @@ commands:
   - after: action
     when: [create]
     run:
-      - echo "Exécution hook de succès"
+      - echo "Exécution hooks de succès"
       - /etc/borgmatic/hooks/ntfy-success.sh
+      - /etc/borgmatic/hooks/uptime-kuma-success.sh
 
   - after: error
     run:
-      - echo "Exécution hook d'erreur"
+      - echo "Exécution hooks d'erreur"
       - /etc/borgmatic/hooks/ntfy-error.sh
+      - /etc/borgmatic/hooks/uptime-kuma-error.sh
 # Commandes PostgreSQL/MySQL si nécessaire
 # postgresql_databases:
 #     - name: all