fix(quota): don't let the demo account persist deep_requested (S1, finding 2)

The demo account is shared, so writing deep_requested onto its subscription polluted
shared state (harmless today — the scheduler's deep paths are human-only — but a
future scheduler path trusting the flag would leak quota). Guard the flag WRITE at the
source (not user.is_demo), which also makes deep_turned_on always False for demo, so
the redundant is_demo check on the immediate backfill is removed.

backend/app/routes/channels.py

@@ -391,7 +391,10 @@ def update_channel(
         sub.priority = int(payload["priority"])
     if "hidden" in payload:
         sub.hidden = bool(payload["hidden"])
-    if "deep_requested" in payload:
+    # The demo account is SHARED, so it must not persist deep_requested (a quota-relevant flag) onto
+    # the shared subscription — skip the write entirely for demo, which also keeps deep_turned_on
+    # False so the immediate backfill below can never fire for it.
+    if "deep_requested" in payload and not user.is_demo:
         # Opt this channel into (or out of) full-history backfill. The deep scheduler
         # picks the flag up on its next run; turning it off won't undo already-fetched
         # videos, it just stops further deep paging if nobody else still wants it.
@@ -403,14 +406,8 @@ def update_channel(
     # Opting a channel into full history should give an immediate feed: if we haven't even
     # fetched its recent uploads yet, do that one channel now (cheap) instead of waiting for
     # the scheduler. Deep paging still follows on the scheduler's next run (recent-then-deep).
-    # Not for the demo account: it must not be able to trigger an immediate quota-burning backfill
-    # (the scheduler's deep paths are already human-only). Same guard as channel_detail's enrichment.
-    if (
-        deep_turned_on
-        and channel is not None
-        and channel.recent_synced_at is None
-        and not user.is_demo
-    ):
+    # (deep_turned_on is only ever True for a non-demo user — see the guard above.)
+    if deep_turned_on and channel is not None and channel.recent_synced_at is None:
         with quota.attribute(user.id, quota.QuotaAction.VIDEOS_BACKFILL_RECENT):
             run_recent_backfill(db, [channel], max_channels=1)