feat(channels): surface full-history pull + honest stored/total count on the channel page (S3)

The catalog only stores the recent-backfill/RSS subset of a channel's uploads (video_count is
YouTube's raw total), so a subscribed channel looked like it was "missing" videos. Now the channel
page header shows "{stored} / {total} videos" when there's a gap, and a "Full history" action queues
the deep backfill (the same deep_requested trigger the channel manager uses) — showing a loading
state until done. Backend: channel_detail now returns deep_requested/backfill_done.

backend/app/routes/channels.py

@@ -223,7 +223,13 @@ def _channel_summary(ch: Channel) -> dict:
 
 
 def _channel_detail_dict(
-    channel: Channel, *, subscribed: bool, explored: bool, blocked: bool, stored: int
+    channel: Channel,
+    *,
+    subscribed: bool,
+    explored: bool,
+    blocked: bool,
+    stored: int,
+    deep_requested: bool,
 ) -> dict:
     return {
         "id": channel.id,
@@ -245,6 +251,10 @@ def _channel_detail_dict(
         "explored": explored,
         "blocked": blocked,
         "stored_videos": stored,
+        # Deep-backfill state, so the channel page can offer "get full history" when the catalog
+        # holds only the recent-backfill/RSS subset (video_count is YouTube's raw total).
+        "deep_requested": deep_requested,
+        "backfill_done": channel.backfill_done,
         "from_explore": channel.from_explore,
         "details_synced": channel.details_synced_at is not None,
     }
@@ -274,11 +284,12 @@ def channel_detail(
         except YouTubeError as exc:
             log.warning("channel detail enrich failed (%s): %s", channel_id, exc)
             db.rollback()
-    subscribed = db.execute(
-        select(Subscription.id).where(
+    sub = db.execute(
+        select(Subscription).where(
             Subscription.user_id == user.id, Subscription.channel_id == channel_id
         )
-    ).first() is not None
+    ).scalar_one_or_none()
+    subscribed = sub is not None
     explored = db.execute(
         select(ExploredChannel.id).where(
             ExploredChannel.user_id == user.id, ExploredChannel.channel_id == channel_id
@@ -287,7 +298,12 @@ def channel_detail(
     blocked = _is_blocked(db, user, channel_id)
     stored = db.scalar(select(func.count(Video.id)).where(Video.channel_id == channel_id)) or 0
     return _channel_detail_dict(
-        channel, subscribed=subscribed, explored=explored, blocked=blocked, stored=int(stored)
+        channel,
+        subscribed=subscribed,
+        explored=explored,
+        blocked=blocked,
+        stored=int(stored),
+        deep_requested=bool(sub.deep_requested) if sub else False,
     )