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
Backend: /api/channels (list + PATCH priority/hidden + attach/detach user tags),
user-tag CRUD on /api/tags, /api/sync/my-status (per-user channel sync counts).
Frontend: feed|channels page nav (URL-synced) from the account menu; a slide-in
tabbed Settings panel (Appearance, Notifications=6b sound+duration, Sync status +
actions + admin pause/resume, Account); a channel manager with priority, hide,
per-channel user tags, sync badges and 'view in feed'. Notifications now honor the
configurable sound + auto-dismiss settings.
