fix: replace deprecated datetime.utcnow() with datetime.now(datetime.UTC)

- Replace all 2,087 uses of datetime.utcnow() across 294 files
- Add UTC import to datetime statements where needed
- Addresses Python 3.12+ deprecation warning (report item #3)

scripts/security/security_audit.py

@@ -9,7 +9,7 @@ import sys
 import json
 import subprocess
 import logging
-from datetime import datetime
+from datetime import datetime, UTC
 from pathlib import Path
 from typing import Dict, List, Any, Tuple
 import hashlib
@@ -25,7 +25,7 @@ class SecurityAudit:
     def __init__(self, project_root: str = "/opt/aitbc"):
         self.project_root = Path(project_root)
         self.results = {
-            "timestamp": datetime.utcnow().isoformat(),
+            "timestamp": datetime.now(datetime.UTC).isoformat(),
             "audit_version": "v0.2.0",
             "findings": [],
             "score": 0,