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/reader/takeout.py

@@ -320,7 +320,10 @@ def stage(db: Session, user: User, parsed: Parsed) -> None:
 
 
 # How many unknown Watch later videos one user may have looked up on the shared API budget in a day:
-# two full Watch later lists (two units a hundred ids). The rest wait for the next day's runs.
+# two full Watch later lists (two units a hundred ids). The rest wait for the next day's runs. Both
+# shares pace the work, they do not bound its cost: they are held in memory, so a restart starts
+# them afresh, which only brings the rest forward — every video is asked about once (a looked-up
+# row is marked), and `quota.backfill_reserve_reached` still guards the shared budget.
 _lookups = RateLimiter(max_events=2 * WATCH_LATER_MAX, window_seconds=24 * 3600)
 # How many watched videos' lengths one user may ask about in a day for the "watched but not followed"
 # list (a unit a fifty). A share of its own: a large history asks for thousands, and on the Watch