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

frontend/src/lib/api.ts

@@ -144,6 +144,9 @@ export interface MyStatus {
   channels_deep_done: number;
   channels_recent_pending: number;
   channels_deep_pending: number;
+  channels_deep_requested: number;
+  deep_pending_count: number;
+  deep_eta_seconds: number;
   my_videos: number;
   quota_used_today: number;
   quota_remaining_today: number;
@@ -160,6 +163,7 @@ export interface ManagedChannel {
   stored_videos: number;
   priority: number;
   hidden: boolean;
+  deep_requested: boolean;
   tag_ids: number[];
   details_synced: boolean;
   recent_synced: boolean;
@@ -184,8 +188,12 @@ export const api = {
   // --- channel manager ---
   myStatus: (): Promise<MyStatus> => req("/api/sync/my-status"),
   channels: (): Promise<ManagedChannel[]> => req("/api/channels"),
-  updateChannel: (id: string, patch: { priority?: number; hidden?: boolean }) =>
-    req(`/api/channels/${id}`, { method: "PATCH", body: JSON.stringify(patch) }),
+  updateChannel: (
+    id: string,
+    patch: { priority?: number; hidden?: boolean; deep_requested?: boolean }
+  ) => req(`/api/channels/${id}`, { method: "PATCH", body: JSON.stringify(patch) }),
+  deepAll: (on = true) =>
+    req(`/api/sync/deep-all?on=${on}`, { method: "POST" }),
   attachChannelTag: (id: string, tagId: number) =>
     req(`/api/channels/${id}/tags`, { method: "POST", body: JSON.stringify({ tag_id: tagId }) }),
   detachChannelTag: (id: string, tagId: number) =>