core/video/discovery_gaps.py — two pure diffs powering the 'what am I missing' rails:
collection_gaps (franchise entries you don't own, in collection order) and
filmography_gaps (a person's titles you don't own, deduped, kind/vote-filtered, ranked
by popularity). No I/O — the API wires owned-ids/collection-items/person-credits in.
9 tests.
