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/router/engine.py

@@ -1,4 +1,4 @@
-import uuid
+import uuid
 import datetime
 from typing import List, Dict, Any, Optional
 from app.models.schemas import (
@@ -64,7 +64,7 @@ def route_request(
 ) -> RoutingDecision:
     req_id = request_id or str(uuid.uuid4())
     decision_id = f"dec_{uuid.uuid4().hex[:12]}"
-    now_iso = datetime.datetime.utcnow().isoformat()
+    now_iso = datetime.datetime.now(datetime.timezone.utc).isoformat()
 
     # 1. Rule evaluation
     rule_action, rule_target, rule_name = None, None, None