fix(datetime): replace deprecated utcnow() with timezone-aware equivalent (#15)

- Replaced datetime.utcnow() with datetime.now(timezone.utc) in router and observability modules
- Preserves backwards compatibility with Python 3.10+
- Fixes #14

backend/app/observability/events.py

@@ -1,4 +1,4 @@
-import json
+import json
 import logging
 from typing import Dict, Any, Optional
 import datetime
@@ -27,7 +27,7 @@ def log_router_event(
     payload = {
         "event": event_name,
         "request_id": request_id,
-        "timestamp": datetime.datetime.utcnow().isoformat(),
+        "timestamp": datetime.datetime.now(datetime.timezone.utc).isoformat(),
         "model": model,
         "provider": provider,
         "duration_ms": duration_ms,