Already-matched movies predate the tmdb_collection_id column, so the collection gap rails
were empty (only newly-enriched movies got the id). Add a self-healing backfill: each
/discover/gaps load fills the franchise id for up to 20 owned movies missing it
(eng.movie_collection reuses the matcher's belongs_to_collection read), recording 0 for
movies with no franchise so they're not re-checked. The 'no-franchise' 0 is excluded from
the rails. Backfill is wrapped/isolated so it can never break the gap response. Over a few
Discover visits the whole library fills in and 'Complete the <franchise>' rails populate.
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