fix(channels): reflect shared deep-queue state in the full-history chip

deep_requested is per-subscription, but deep backfill is channel-wide and shared:
once any subscriber requests full history, the whole back-catalog arrives for
everyone. The chip only looked at the current user's flag, so a second subscriber
saw a misleading 'get full history' on a channel already queued by someone else.
Add a channel-level deep_in_queue to /api/channels and show an informational
'full history queued' badge (vs the owner's cancelable button) in that case.

backend/app/routes/channels.py

@@ -36,6 +36,22 @@ def list_channels(
         ).all():
             stored[cid] = count
 
+    # Channels already in the *global* deep-backfill queue — i.e. at least one user (any
+    # user) has requested full history and it isn't done yet. Their whole back-catalogue is
+    # coming for everyone (video data is shared), so the UI shouldn't offer "get full
+    # history" to a user who simply hasn't personally opted in.
+    deep_queued: set[str] = set()
+    if channel_ids:
+        for (cid,) in db.execute(
+            select(Subscription.channel_id)
+            .where(
+                Subscription.channel_id.in_(channel_ids),
+                Subscription.deep_requested.is_(True),
+            )
+            .distinct()
+        ).all():
+            deep_queued.add(cid)
+
     # The user's personal tag links, grouped by channel.
     tags_by_channel: dict[str, list[int]] = {}
     for cid, tag_id in db.execute(
@@ -57,6 +73,7 @@ def list_channels(
             "priority": sub.priority,
             "hidden": sub.hidden,
             "deep_requested": sub.deep_requested,
+            "deep_in_queue": ch.id in deep_queued and not ch.backfill_done,
             "tag_ids": tags_by_channel.get(ch.id, []),
             "details_synced": ch.details_synced_at is not None,
             "recent_synced": ch.recent_synced_at is not None,