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/app/routes/me.py

@@ -40,7 +40,7 @@ def my_accounts(
     """The accounts that have authenticated in this browser session (switchable without a
     new Google sign-in). The active one is flagged."""
     ids = request.session.get("account_ids") or [user.id]
-    rows = {u.id: u for u in db.query(User).filter(User.id.in_(ids)).all()}
+    rows = {u.id: u for u in db.execute(select(User).where(User.id.in_(ids))).scalars()}
     out = []
     for uid in ids:  # preserve recency order from the session
         u = rows.get(uid)