fix(catalog): a follow from YouTube keeps an explored channel, and the list counts only what can come

A subscription pulled in from YouTube left an explored channel flagged as an exploration's, so its
later videos came in hidden from the Library with their watched marks waiting for good; a subscribe
landing while an exploration's videos came in left those flagged too. One step, settle_explored,
now holds the rule by who follows the channel now: the subscribe here, the subscription import, the
watched-channel videos and the explore page all end with it. Migration 0116 clears what the gap
left behind.

The list's "may still grow" count no longer counts a channel it already shows from a stub row, and
the comments on the cleanup's keep rule and on the in-memory daily shares say what they do.

backend/app/sync/explore.py

@@ -14,6 +14,7 @@ from sqlalchemy import delete, or_, select
 from sqlalchemy.orm import Session
 
 from app import logs, sysconfig
+from app.catalog import service as catalog_service
 from app.catalog.models import Channel, ExploredChannel, SearchFind, Subscription, Video
 from app.reader import queries as reader_queries
 from app.sync.videos import insert_stubs, stub_from_playlist_item, apply_video_details
@@ -91,6 +92,7 @@ def explore_ingest_page(
             stub["via_explore"] = channel.from_explore
             stubs.append(stub)
     inserted = insert_stubs(db, stubs)
+    catalog_service.settle_explored(db, channel)
     self_enriched = _enrich_new(db, yt, [s["id"] for s in stubs])
     log.info(
         "explore ingest channel=%s ingested=%s enriched=%s", channel.id, inserted, self_enriched