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/notifications.ts

@@ -24,7 +24,12 @@ export type VideoWatchedMeta = {
   videoId: string;
   title: string;
 };
-export type NotifMeta = VideoHiddenMeta | VideoWatchedMeta;
+export type ChannelSubscribedMeta = {
+  kind: "channel-subscribed";
+  channelId: string;
+  channelName: string;
+};
+export type NotifMeta = VideoHiddenMeta | VideoWatchedMeta | ChannelSubscribedMeta;
 
 export interface Notification {
   id: number;
@@ -239,7 +244,13 @@ export function remove(id: number): void {
  * now-stale "Hidden/Watched X" notice disappears instead of lingering with a dead action. */
 export function resolveVideo(videoId: string): void {
   const before = items.length;
-  items = items.filter((n) => n.meta?.videoId !== videoId);
+  items = items.filter(
+    (n) =>
+      !(
+        (n.meta?.kind === "video-hidden" || n.meta?.kind === "video-watched") &&
+        n.meta.videoId === videoId
+      )
+  );
   if (items.length !== before) emit();
 }