Replace deep-OFFSET paging (which scans-and-discards skipped rows on the
233k-row catalog and shifts/duplicates rows as the scheduler ingests mid-
scroll) with stable keyset pagination over an opaque cursor.
A single sort registry drives both the ORDER BY and a generic, NULL-aware
keyset WHERE so they can never drift, with Video.id as the unique final
tiebreaker. Covers every sort: newest/oldest, views, duration, title,
subscribers (joined), priority (coalesced multi-key) and the seeded
shuffle. The response now returns next_cursor instead of offset.
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