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/catalog/models.py

@@ -71,10 +71,12 @@ class Channel(Base, TimestampMixin):
         Boolean, default=False, server_default="false", index=True
     )
     # True when this channel exists only because a user EXPLORED it (opened its page without
-    # subscribing) — an ephemeral, per-explorer addition. The cleanup job hard-deletes such a
-    # channel (and its via_explore videos) once nobody keeps it (no subscription, no live
-    # ExploredChannel row, no per-user state) after a grace period. Cleared when someone
-    # subscribes (the "keep" signal promotes it to organic).
+    # subscribing) — an ephemeral, per-explorer addition. After a grace period the cleanup job
+    # hard-deletes its via_explore videos nobody keeps (no subscription, no live ExploredChannel
+    # row, no per-user state), and then the channel row itself once it holds no videos — unless a
+    # Takeout watch history shows it watched: the "watched but not followed" list names it from
+    # that row. Cleared when someone subscribes, here or on YouTube (the "keep" signal promotes it
+    # to organic).
     from_explore: Mapped[bool] = mapped_column(
         Boolean, default=False, server_default="false", index=True
     )
@@ -235,8 +237,9 @@ class ExploredChannel(Base, TimestampMixin):
     Exploring an un-subscribed channel ingests its recent uploads (flagged via_explore) so the
     user can browse them, but keeps them out of everyone else's catalog. This row (a) gates the
     explorer's access to those ephemeral videos in the feed query and (b) drives the grace-period
-    purge: a from_explore channel with no live ExploredChannel row (and no subscription / no
-    per-user state on its videos) is hard-deleted by the cleanup job. Subscribing is the "keep"
+    purge: of a from_explore channel with no live ExploredChannel row (and no subscription) the
+    cleanup job hard-deletes the videos without per-user state, then the channel once it holds none
+    and no Takeout watch history shows it watched. Subscribing is the "keep"
     signal — it promotes the channel to organic and these rows become irrelevant. `created_at`
     (from TimestampMixin) is the grace clock; a re-visit refreshes it."""
 

backend/app/catalog/service.py

@@ -96,13 +96,24 @@ def record_subscription(db: Session, user: User, channel: Channel, yt_subscripti
             subscribed_at=datetime.now(timezone.utc),
         )
     )
-    if channel.from_explore:
-        channel.from_explore = False
-        db.execute(
-            update(Video)
-            .where(Video.channel_id == channel.id, Video.via_explore.is_(True))
-            .values(via_explore=False)
-        )
+    db.flush()
+    settle_explored(db, channel)
+
+
+def settle_explored(db: Session, channel: Channel) -> None:
+    """Hold the rule that a channel's videos are an exploration's only while nobody follows it,
+    decided by who follows it now, not by the flag held in memory: a subscribe in another request
+    may have cleared that while YouTube answered this one, and the videos that came in meanwhile
+    would stay flagged. A channel someone follows, however the follow came (here or pulled in from
+    YouTube), is the catalog's own, and so are its videos."""
+    if channel.from_explore and not queries.followed(db, channel.id):
+        return
+    channel.from_explore = False
+    db.execute(
+        update(Video)
+        .where(Video.channel_id == channel.id, Video.via_explore.is_(True))
+        .values(via_explore=False)
+    )
     db.flush()