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/scheduler.py

@@ -106,10 +106,6 @@ def apply_interval(db, job_id: str, minutes: int) -> int:
     return minutes
 
 
-def _now_iso() -> str:
-    return datetime.now(timezone.utc).isoformat()
-
-
 def _record(name: str, **fields) -> None:
     with _activity_lock:
         _activity.setdefault(name, {}).update(fields)
@@ -187,7 +183,7 @@ def _job(name: str, fn) -> None:
     db = SessionLocal()
     actor_id = _manual_actor.get()  # set only for a manual "run now" trigger
     _record(
-        name, last_started=_now_iso(), last_error=None, progress=None
+        name, last_started=datetime.now(timezone.utc).isoformat(), last_error=None, progress=None
     )  # running already claimed
 
     def sink(current, total, phase):
@@ -199,7 +195,7 @@ def _job(name: str, fn) -> None:
             _record(
                 name,
                 status="skipped",
-                last_finished=_now_iso(),
+                last_finished=datetime.now(timezone.utc).isoformat(),
                 last_result="paused",
                 result=_PAUSED,
                 progress=None,
@@ -216,7 +212,7 @@ def _job(name: str, fn) -> None:
         _record(
             name,
             status="ok",
-            last_finished=_now_iso(),
+            last_finished=datetime.now(timezone.utc).isoformat(),
             last_result=summary,
             result=data,
             progress=None,
@@ -236,7 +232,7 @@ def _job(name: str, fn) -> None:
         _record(
             name,
             status="error",
-            last_finished=_now_iso(),
+            last_finished=datetime.now(timezone.utc).isoformat(),
             last_error=err,
             result=failure,
             progress=None,