refactor(backend): select() and an inline now everywhere, held by a test

CLAUDE.md's canonical table names SQLAlchemy 2.0 select() and an inline
datetime.now(timezone.utc), and both had drifted: five db.query(...) calls
remained in routes/auth.py and routes/me.py (seven before the demo went),
and four private now helpers stood in sync/videos.py, utils.py (imported
as _now by explore and maintenance), scheduler.py (_now_iso) and the e2e
seed. All are the canonical form now.

Ruff cannot ban a method call on an instance or a function by name, so
tests/test_canonical_patterns.py reads every app module's AST and fails on
a .query(...) call or a now/_now/now_utc helper — seen red on a planted file.

scripts/e2e_seed.py

@@ -45,10 +45,6 @@ if not PASSWORD:
 N_SUBS = 3
 
 
-def _now() -> datetime:
-    return datetime.now(timezone.utc)
-
-
 def main() -> None:
     db = SessionLocal()
     try:
@@ -99,7 +95,11 @@ def main() -> None:
 
         db.execute(delete(Subscription).where(Subscription.user_id == user.id))
         for cid in channel_ids:
-            db.add(Subscription(user_id=user.id, channel_id=cid, subscribed_at=_now()))
+            db.add(
+                Subscription(
+                    user_id=user.id, channel_id=cid, subscribed_at=datetime.now(timezone.utc)
+                )
+            )
         db.commit()
 
         # 3. Deterministic watch state (reset each run):
@@ -133,7 +133,7 @@ def main() -> None:
                         video_id=vid.id,
                         status="new",
                         position_seconds=42,
-                        progress_updated_at=_now(),
+                        progress_updated_at=datetime.now(timezone.utc),
                     )
                 )
                 in_progress = vid.id
@@ -146,7 +146,7 @@ def main() -> None:
                         user_id=user.id,
                         video_id=wv.id,
                         status="watched",
-                        watched_at=_now(),
+                        watched_at=datetime.now(timezone.utc),
                     )
                 )
                 watched = wv.id