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/tests/test_catalog_service.py

@@ -22,6 +22,7 @@ from app.catalog.models import (
 )
 from app.reader import queries as reader_queries
 from app.reader import service as reader_service
+from app.sync import explore as sync_explore
 
 
 def _channel(db, cid="UCc", **kw):
@@ -141,6 +142,30 @@ def test_subscribing_keeps_an_explored_channel_and_its_videos(db, make_user):
     assert queries.subscription(db, user, "UCc") is None
 
 
+def test_exploring_a_channel_someone_follows_brings_its_uploads_in_as_the_catalogs_own(
+    db, make_user, monkeypatch
+):
+    follower, explorer = make_user(), make_user()
+    ch = _channel(db, uploads_playlist_id="UUc")
+    db.add(Subscription(user_id=follower.id, channel_id="UCc"))
+    db.commit()
+    service.explore(db, explorer, ch)
+    db.commit()
+
+    class _Yt:
+        def get_playlist_items(self, playlist_id, page_token):
+            item = {"contentDetails": {"videoId": "v1", "videoPublishedAt": "2026-05-01T00:00:00Z"}}
+            return {"items": [item]}
+
+    monkeypatch.setattr(sync_explore, "_enrich_new", lambda session, yt, ids: 0)
+    sync_explore.explore_ingest_page(db, _Yt(), ch)
+    db.commit()
+    db.expire_all()
+    # The channel is followed, so not the exploration's: a video flagged as one would stay flagged
+    # (subscribing clears only an explored channel's) and its watched marks would wait for good.
+    assert (ch.from_explore, db.get(Video, "v1").via_explore) == (False, False)
+
+
 def test_a_block_is_once_and_ends_the_exploration(db, make_user):
     user = make_user()
     ch = _channel(db)