feat(m5d): demand-driven deep backfill + per-user ETA

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/app/routes/channels.py

@@ -54,6 +54,7 @@ def list_channels(
             "stored_videos": stored.get(ch.id, 0),
             "priority": sub.priority,
             "hidden": sub.hidden,
+            "deep_requested": sub.deep_requested,
             "tag_ids": tags_by_channel.get(ch.id, []),
             "details_synced": ch.details_synced_at is not None,
             "recent_synced": ch.recent_synced_at is not None,
@@ -86,8 +87,18 @@ def update_channel(
         sub.priority = int(payload["priority"])
     if "hidden" in payload:
         sub.hidden = bool(payload["hidden"])
+    if "deep_requested" in payload:
+        # Opt this channel into (or out of) full-history backfill. The deep scheduler
+        # picks the flag up on its next run; turning it off won't undo already-fetched
+        # videos, it just stops further deep paging if nobody else still wants it.
+        sub.deep_requested = bool(payload["deep_requested"])
     db.commit()
-    return {"id": channel_id, "priority": sub.priority, "hidden": sub.hidden}
+    return {
+        "id": channel_id,
+        "priority": sub.priority,
+        "hidden": sub.hidden,
+        "deep_requested": sub.deep_requested,
+    }
 
 
 @router.post("/{channel_id}/tags")