Add conditions field for Uptime Kuma v2 compatibility

Uptime Kuma v2 requires a non-null conditions field when creating
monitors. Adding empty array to satisfy the constraint.

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

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

backend/services/kuma_client.py

@@ -203,6 +203,7 @@ class UptimeKumaClient:
                 "interval": monitor.interval,
                 "maxretries": monitor.retries,
                 "retryInterval": monitor.retry_interval,
+                "conditions": [],  # Required for Uptime Kuma v2
             }
 
             # Add type-specific fields