style: apply ruff format across backend + scripts (line-length 100)

Mechanical, whole-tree `ruff format` pass — no behaviour change. Recorded in
.git-blame-ignore-revs (next commit) so git blame skips it.

backend/app/demo.py

@@ -4,6 +4,7 @@ Shared by the admin's manual reset endpoint (routes/admin.py) and the scheduled
 (scheduler.py). Lives in the service layer, not routes, so the scheduler can call it without an
 `app.routes -> app.scheduler` import cycle (the reason its import used to be lazy).
 """
+
 from sqlalchemy import delete, func, select
 from sqlalchemy.orm import Session
 
@@ -49,13 +50,17 @@ def _seed_demo_subscriptions(db: Session, demo: User, n: int = 14) -> int:
     Channel manager isn't empty — gives visitors something to explore and matches the landing-page
     screenshot. Idempotent: clears the demo's subscriptions first, then re-adds the top N."""
     db.execute(delete(Subscription).where(Subscription.user_id == demo.id))
-    top = db.execute(
-        select(Video.channel_id)
-        .where(Video.channel_id.is_not(None))
-        .group_by(Video.channel_id)
-        .order_by(func.count().desc())
-        .limit(n)
-    ).scalars().all()
+    top = (
+        db.execute(
+            select(Video.channel_id)
+            .where(Video.channel_id.is_not(None))
+            .group_by(Video.channel_id)
+            .order_by(func.count().desc())
+            .limit(n)
+        )
+        .scalars()
+        .all()
+    )
     for channel_id in top:
         db.add(Subscription(user_id=demo.id, channel_id=channel_id))
     return len(top)