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/utils.py

@@ -1,7 +1,7 @@
 """Small cross-cutting helpers shared across modules (kept dependency-light on purpose)."""
 
 import re
-from datetime import date, datetime, timezone
+from datetime import date
 
 
 def iso(dt: date | None) -> str | None:
@@ -55,11 +55,6 @@ def canonical_email(value: str | None) -> str:
     return f"{local}@gmail.com" if local else email
 
 
-def now_utc() -> datetime:
-    """Current time as a timezone-aware UTC datetime."""
-    return datetime.now(timezone.utc)
-
-
 # The shortest last word that is prefix-matched. A 1–2 letter prefix matches a large share of the
 # library and ts_rank then scores every match: "m" took ~5–8 s on prod against ~100 ms for a whole
 # word (E31 S4e). A shorter last word is matched whole instead ("tv", "4k" still find themselves).