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/auth.py

@@ -399,7 +399,7 @@ async def callback(
             return RedirectResponse(url=_access_notice_url("requested", email, req.invite))
         return RedirectResponse(url=_access_notice_url("denied"))
 
-    user = db.query(User).filter(User.google_sub == sub).one_or_none()
+    user = db.execute(select(User).where(User.google_sub == sub)).scalar_one_or_none()
     if user is None:
         # No account carries this Google identity yet. If one already exists for this MAILBOX —
         # e.g. an email+password registration — adopt the Google identity onto it instead of
@@ -412,7 +412,11 @@ async def callback(
         # (ids 13 and 14). Exact spelling still wins when both exist, so an account that already
         # matches literally is never passed over for a canonical sibling.
         canon = canonical_email(email)
-        candidates = db.query(User).filter(User.email_canonical == canon).order_by(User.id).all()
+        candidates = (
+            db.execute(select(User).where(User.email_canonical == canon).order_by(User.id))
+            .scalars()
+            .all()
+        )
         existing = next((u for u in candidates if u.email == email), None) or next(
             iter(candidates), None
         )
@@ -516,7 +520,9 @@ def _sync_google_email(db: Session, user: User, email: str) -> None:
         return
     if canonical_email(user.email) == canonical_email(email):
         return  # same mailbox, their spelling — leave it alone
-    clash = db.query(User).filter(User.email == email, User.id != user.id).one_or_none()
+    clash = db.execute(
+        select(User).where(User.email == email, User.id != user.id)
+    ).scalar_one_or_none()
     if clash is None:
         user.email = email
 
@@ -575,7 +581,9 @@ def _complete_link(
     user = db.get(User, link_uid)
     if user is None:
         return RedirectResponse(url="/?link=error")
-    other = db.query(User).filter(User.google_sub == sub, User.id != user.id).one_or_none()
+    other = db.execute(
+        select(User).where(User.google_sub == sub, User.id != user.id)
+    ).scalar_one_or_none()
     if other is not None:
         return RedirectResponse(url="/?link=conflict")
     if user.google_sub is not None and user.google_sub != sub: