Replace deprecated datetime.utcnow with shared naive-UTC helper

All 29 call sites now use app.timeutil.utcnow(), which returns naive
UTC via the non-deprecated API. Semantics are unchanged on purpose:
every datetime column is TIMESTAMP WITHOUT TIME ZONE, so going aware
here would poison naive/aware comparisons. The TIMESTAMPTZ migration
(Phase 2) now has a single place to change. (BE-01)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/app/timeutil.py

@@ -0,0 +1,14 @@
+from datetime import datetime, timezone
+
+
+def utcnow() -> datetime:
+    """Naive UTC now, via the non-deprecated API.
+
+    Every datetime column in the schema is TIMESTAMP WITHOUT TIME ZONE, so the
+    whole app speaks naive-UTC. Replacing datetime.utcnow() call sites with
+    this helper removes the Python 3.12 deprecation without changing any
+    stored value or comparison. When the columns move to TIMESTAMPTZ
+    (Phase 2), this becomes `datetime.now(timezone.utc)` and the type change
+    happens in exactly one place.
+    """
+    return datetime.now(timezone.utc).replace(tzinfo=None)