Per-user opt-in to full-history (deep) backfill so a new user's unique
channels no longer trigger a big shared-quota burst.
- migration 0007: Subscription.deep_requested (default false); seed admins'
existing subscriptions to preserve today's global-backfill behaviour
- run_deep_backfill is now demand-driven: only channels at least one user has
requested are deep-backfilled; recent backfill stays unconditional (cheap)
- estimate_deep_backfill ETA helper (quota-bound) surfaced in /api/sync/my-status
- POST /api/sync/deep-all to opt all my channels in; PATCH channels accepts
deep_requested
- UI: per-channel Full history toggle, Backfill everything action, deep
progress + ETA in Channels header and Settings - Sync
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