feat(channels): kick recent backfill immediately on 'full history' opt-in

When a user requests full history for a channel whose recent uploads aren't
fetched yet, run a one-channel recent backfill synchronously in the request so
the feed populates at once instead of waiting for the scheduler. Deep paging
still follows on the scheduler (recent-then-deep). The deep-toggle mutation now
also refreshes my-status and the feed.

backend/app/routes/channels.py

@@ -8,6 +8,7 @@ from sqlalchemy.orm import Session
 from app.auth import current_user, has_write_scope
 from app.db import get_db
 from app.models import Channel, ChannelTag, Subscription, Tag, User, Video
+from app.sync.runner import run_recent_backfill
 from app.youtube.client import YouTubeClient, YouTubeError
 
 router = APIRouter(prefix="/api/channels", tags=["channels"])
@@ -84,6 +85,8 @@ def update_channel(
     db: Session = Depends(get_db),
 ) -> dict:
     sub = _user_subscription(db, user, channel_id)
+    channel = db.get(Channel, channel_id)
+    deep_turned_on = False
     if "priority" in payload:
         sub.priority = int(payload["priority"])
     if "hidden" in payload:
@@ -92,13 +95,23 @@ def update_channel(
         # 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.
-        sub.deep_requested = bool(payload["deep_requested"])
+        want = bool(payload["deep_requested"])
+        deep_turned_on = want and not sub.deep_requested
+        sub.deep_requested = want
     db.commit()
+
+    # 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).
+    if deep_turned_on and channel is not None and channel.recent_synced_at is None:
+        run_recent_backfill(db, [channel], max_channels=1)
+
     return {
         "id": channel_id,
         "priority": sub.priority,
         "hidden": sub.hidden,
         "deep_requested": sub.deep_requested,
+        "recent_synced": channel.recent_synced_at is not None if channel else None,
     }