When recent backfill stopped partway through a page (age cutoff or count cap),
it stored the *next* page token as the deep-backfill cursor, so the older items
remaining on that same page were collected by neither pass — a channel could
report backfill_done while silently missing a band of videos right around the
365-day cutoff. Now resume deep from the page we stopped on (re-fetched; inserts
are idempotent). Verified: Pánczél went 200 -> 209 stored (= full uploads
playlist) after re-backfill.
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