A channel whose stored uploads already meet or exceed YouTube's advertised
video_count holds its whole history, but backfill_done could stay false forever
when the deep cursor never reached the end (e.g. a small channel that was never
deep-requested, so the demand-driven deep job never ran). Such channels nagged
as 'needs full history' despite having every video. Add reconcile_full_history()
(idempotent, no quota) and run it at the end of each recent/deep backfill cycle
so backfill_done self-heals.
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