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.

backend/tests/test_maintenance_lifecycle.py

@@ -7,14 +7,13 @@ trigger, and an upcoming premiere that never went live is. YouTube is a fake tha
 videos.list from a dict.
 """
 
-from datetime import timedelta
+from datetime import datetime, timedelta, timezone
 
 import pytest
 from sqlalchemy import select
 
 from app.models import Channel, Notification, Playlist, PlaylistItem, Video
 from app.sync import maintenance
-from app.utils import now_utc
 
 
 class _YT:
@@ -41,11 +40,14 @@ def channel(db):
 
 def test_a_flagged_video_that_answers_again_is_unhidden(db, channel):
     v = Video(
-        id="v1", channel_id=channel.id, unavailable_since=now_utc(), unavailable_reason="removed"
+        id="v1",
+        channel_id=channel.id,
+        unavailable_since=datetime.now(timezone.utc),
+        unavailable_reason="removed",
     )
     db.add(v)
     db.commit()
-    out = maintenance._recheck_flagged(db, _YT({"v1": _item("v1")}), now_utc())
+    out = maintenance._recheck_flagged(db, _YT({"v1": _item("v1")}), datetime.now(timezone.utc))
     db.refresh(v)
     assert v.unavailable_since is None and v.unavailable_reason is None
     assert out == {"checked": 1, "recovered": 1, "deleted": 0, "notified": 0}
@@ -55,12 +57,13 @@ def test_a_video_still_missing_inside_its_grace_period_is_kept_hidden(db, channe
     v = Video(
         id="v1",
         channel_id=channel.id,
-        unavailable_since=now_utc() - timedelta(days=maintenance.DEFAULT_GRACE_DAYS - 1),
+        unavailable_since=datetime.now(timezone.utc)
+        - timedelta(days=maintenance.DEFAULT_GRACE_DAYS - 1),
         unavailable_reason="removed",
     )
     db.add(v)
     db.commit()
-    out = maintenance._recheck_flagged(db, _YT({}), now_utc())
+    out = maintenance._recheck_flagged(db, _YT({}), datetime.now(timezone.utc))
     assert db.get(Video, "v1") is not None
     assert out["deleted"] == 0
 
@@ -69,7 +72,7 @@ def test_past_the_grace_period_it_is_deleted_and_each_user_who_kept_it_told_once
     db, channel, make_user
 ):
     a, b = make_user(), make_user()
-    gone_since = now_utc() - timedelta(days=maintenance.DEFAULT_GRACE_DAYS + 1)
+    gone_since = datetime.now(timezone.utc) - timedelta(days=maintenance.DEFAULT_GRACE_DAYS + 1)
     for vid in ("v1", "v2"):
         db.add(
             Video(
@@ -92,7 +95,7 @@ def test_past_the_grace_period_it_is_deleted_and_each_user_who_kept_it_told_once
         ]
     )
     db.commit()
-    out = maintenance._recheck_flagged(db, _YT({}), now_utc())
+    out = maintenance._recheck_flagged(db, _YT({}), datetime.now(timezone.utc))
     db.expire_all()
     assert db.get(Video, "v1") is None and db.get(Video, "v2") is None
     assert db.execute(select(PlaylistItem)).scalars().all() == []  # the cascade
@@ -118,7 +121,7 @@ def test_rolling_revalidation_flags_the_missing_and_keeps_a_live_stream(db, chan
     )
     db.commit()
     yt = _YT({"ok": _item("ok"), "live": _item("live", liveBroadcastContent="live")})
-    out = maintenance._revalidate_rolling(db, yt, now_utc())
+    out = maintenance._revalidate_rolling(db, yt, datetime.now(timezone.utc))
     db.expire_all()
     assert db.get(Video, "gone").unavailable_reason == "removed"
     assert db.get(Video, "ok").unavailable_since is None
@@ -150,7 +153,7 @@ def test_a_run_asks_youtube_about_each_video_once(db, channel, monkeypatch, make
             Video(
                 id="v1",
                 channel_id=channel.id,
-                unavailable_since=now_utc(),
+                unavailable_since=datetime.now(timezone.utc),
                 unavailable_reason="removed",
             ),
             Video(id="v2", channel_id=channel.id),
@@ -163,8 +166,8 @@ def test_a_run_asks_youtube_about_each_video_once(db, channel, monkeypatch, make
 
 
 def test_an_upcoming_premiere_counts_as_abandoned_only_when_it_never_went_live():
-    past = (now_utc() - timedelta(days=3)).isoformat()
-    soon = (now_utc() - timedelta(days=1)).isoformat()
+    past = (datetime.now(timezone.utc) - timedelta(days=3)).isoformat()
+    soon = (datetime.now(timezone.utc) - timedelta(days=1)).isoformat()
 
     def item(**live):
         return {"snippet": {"liveBroadcastContent": "upcoming"}, "liveStreamingDetails": live}