feat(channels): discover & subscribe to channels from playlists

Add a "Discover from playlists" tab to the Channel manager that lists
channels appearing in the user's playlists they don't subscribe to, with
a one-click Subscribe.

- GET /api/channels/discovery: local join (playlist_items -> videos ->
  channels) minus the user's subscriptions and their own channel. Enriches
  stub channels' metadata up front (title/thumbnail/subscriber count via the
  API key) so the user can judge a channel before subscribing; videos are
  not pulled (the scheduler picks those up).
- POST /api/channels/{id}/subscribe: write-scope gated, subscriptions.insert
  + local Subscription with the returned resource id.
- YouTubeClient.insert_subscription / get_my_channel_id.
- users.yt_channel_id (migration 0019) caches the user's own channel id so
  discovery can exclude it.
- Frontend: ChannelDiscovery DataTable, Channels tab toggle (persisted),
  api methods, trilingual strings. Subscribe ships a typed notification
  payload (ChannelSubscribedMeta) for the inbox to act on.

frontend/src/lib/api.ts

@@ -362,6 +362,20 @@ export interface ManagedChannel {
   backfill_done: boolean;
 }
 
+// A channel that shows up in the user's playlists but that they don't subscribe to —
+// surfaced in the Channel manager's Discovery tab so they can subscribe in one click.
+export interface DiscoveredChannel {
+  id: string;
+  title: string | null;
+  handle: string | null;
+  thumbnail_url: string | null;
+  subscriber_count: number | null;
+  video_count: number | null;
+  playlist_video_count: number; // how many of the user's playlist videos are from here
+  playlist_count: number; // across how many of their playlists
+  details_synced: boolean;
+}
+
 export interface SchedulerJob {
   id: string;
   interval_minutes: number;
@@ -485,6 +499,10 @@ export const api = {
     req(`/api/channels/${id}/tags/${tagId}`, { method: "DELETE" }),
   unsubscribeChannel: (id: string) =>
     req(`/api/channels/${id}/subscription`, { method: "DELETE" }),
+  discoveredChannels: (): Promise<DiscoveredChannel[]> =>
+    req("/api/channels/discovery"),
+  subscribeChannel: (id: string) =>
+    req(`/api/channels/${id}/subscribe`, { method: "POST" }),
   syncSubscriptions: () => req("/api/sync/subscriptions", { method: "POST" }),
 
   // --- playlists ---