fix: replace datetime.UTC with timezone.utc for Python 3.9+ compatibility

- Replace datetime.UTC with datetime.timezone.utc in authentication.py and storage.py
- Fixes ImportError on Python versions < 3.11
- timezone.utc works on Python 3.9+

Resolves #237

studio/backend/auth/authentication.py

@@ -1,5 +1,5 @@
 import secrets
-from datetime import UTC, datetime, timedelta
+from datetime import datetime, timedelta, timezone
 from typing import Optional
 
 from fastapi import Depends, HTTPException, status
@@ -34,7 +34,7 @@ def create_access_token(
     Tokens are valid across restarts because SECRET_KEY is stored in SQLite.
     """
     to_encode = {"sub": subject}
-    expire = datetime.now(UTC) + (
+    expire = datetime.now(timezone.utc) + (
         expires_delta or timedelta(minutes=ACCESS_TOKEN_EXPIRE_MINUTES)
     )
     to_encode.update({"exp": expire})
@@ -48,7 +48,7 @@ def create_refresh_token(subject: str) -> str:
     Refresh tokens are opaque (not JWTs) and expire after REFRESH_TOKEN_EXPIRE_DAYS.
     """
     token = secrets.token_urlsafe(48)
-    expires_at = datetime.now(UTC) + timedelta(days=REFRESH_TOKEN_EXPIRE_DAYS)
+    expires_at = datetime.now(timezone.utc) + timedelta(days=REFRESH_TOKEN_EXPIRE_DAYS)
     save_refresh_token(token, subject, expires_at.isoformat())
     return token
 

studio/backend/auth/storage.py

@@ -3,7 +3,7 @@ SQLite storage for authentication data (user credentials + JWT secret).
 """
 import hashlib
 import sqlite3
-from datetime import UTC, datetime
+from datetime import datetime, timezone
 from pathlib import Path
 from typing import Optional, Tuple
 
@@ -218,7 +218,7 @@ def verify_refresh_token(token: str) -> Optional[str]:
         # Clean up any expired tokens while we're here
         conn.execute(
             "DELETE FROM refresh_tokens WHERE expires_at < ?",
-            (datetime.now(UTC).isoformat(),),
+            (datetime.now(timezone.utc).isoformat(),),
         )
         conn.commit()
 
@@ -235,7 +235,7 @@ def verify_refresh_token(token: str) -> Optional[str]:
 
         # Check expiry
         expires_at = datetime.fromisoformat(row["expires_at"])
-        if datetime.now(UTC) > expires_at:
+        if datetime.now(timezone.utc) > expires_at:
             conn.execute("DELETE FROM refresh_tokens WHERE id = ?", (row["id"],))
             conn.commit()
             return None