fix: use timezone-aware datetime in webhook timestamp

Replace deprecated datetime.utcnow() with datetime.now(timezone.utc)
to avoid Python 3.12+ DeprecationWarning.

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

web/paperclip_api.py

@@ -19,7 +19,7 @@ from __future__ import annotations
 
 import logging
 import os
-from datetime import datetime
+from datetime import datetime, timezone
 from typing import Any
 
 import httpx
@@ -104,7 +104,7 @@ async def emit_case_status_webhook(
                 "oldStatus": old_status,
                 "newStatus": new_status,
                 "companyId": company_id,
-                "timestamp": datetime.utcnow().isoformat() + "Z",
+                "timestamp": datetime.now(timezone.utc).isoformat(),
             },
             run_id=run_id,
             timeout=5.0,