style(translation): use datetime.UTC alias (ruff UP017)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/services/interpreter_client.py

@@ -8,7 +8,7 @@ behind a reverse proxy). Full API contract: Scribe note #1347.
 """
 from __future__ import annotations
 
-from datetime import datetime, timezone
+from datetime import UTC, datetime
 from email.utils import parsedate_to_datetime
 
 import requests
@@ -53,8 +53,8 @@ def _parse_retry_after(resp) -> float | None:
     if when is None:
         return None
     if when.tzinfo is None:
-        when = when.replace(tzinfo=timezone.utc)
-    return max(0.0, (when - datetime.now(timezone.utc)).total_seconds())
+        when = when.replace(tzinfo=UTC)
+    return max(0.0, (when - datetime.now(UTC)).total_seconds())
 
 
 def health(base_url: str, *, timeout: float = 5.0) -> bool: