fix(reader): the watched-channels list keeps its channels, its shares and its retries

- A channel someone follows: the videos its exploration brings in (the list's
  dialog, the channel page's explore) come in as the catalog's own, not
  flagged via_explore, which subscribing would never clear and which kept
  their watched marks waiting for good.
- The explore cleanup keeps the row of a channel a Takeout history shows
  watched (it still reclaims the videos), so a looked-at channel no longer
  drops off the list.
- The Shorts job takes up only users whose list still has lengths to ask or
  channels to name, by each user's own follows, blocks and dismissals; a
  naming that failed is tried again, and the list says it is not complete.
- The length lookups have a daily share of their own: a large history no
  longer leaves the export's Watch later file waiting a day.
- Only a channel of the user's own history can be put out of the list.
- The dialog's videos have a query root of their own: the list's
  invalidations no longer re-ask (and re-explore) under an open dialog.

backend/app/routes/channels.py

@@ -307,9 +307,9 @@ def explore_channel(
     """Open an un-subscribed channel for browsing. Records the exploration (a per-user,
     grace-clocked row that gates this user's access to the channel's ephemeral videos and
     drives the later cleanup), flags the channel ephemeral unless someone organically follows
-    it, then ingests one page of its uploads (newest-first) marked via_explore + enriched.
-    Returns the next page token so the page can "load more". Guarded against the shared
-    backfill quota reserve."""
+    it, then ingests one page of its uploads (newest-first), enriched — marked via_explore while
+    nobody follows it. Returns the next page token so the page can "load more". Guarded against
+    the shared backfill quota reserve."""
     channel = _channel_or_404(db, channel_id)
     if queries.is_blocked(db, user, channel_id):
         raise refused(403, "channel_blocked")