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/i18n/locales/en/channels.json

@@ -5,6 +5,25 @@
   "syncSubscriptionsHint": "Re-import your subscription list from YouTube — adds channels you've newly followed and drops ones you've unfollowed. The videos themselves keep syncing automatically in the background; this does not re-fetch them.",
   "backfillEverything": "Backfill everything",
   "backfillEverythingHint": "Request full back-catalog backfill for every channel you're subscribed to. Older videos and search become complete as the shared daily quota allows — this can take a while.",
+  "tabs": {
+    "subscribed": "Subscriptions",
+    "discovery": "Discover from playlists"
+  },
+  "discovery": {
+    "intro": "Channels that appear in your playlists but that you don't subscribe to. Subscribe to follow them — their new uploads will start showing in your feed (subscribing costs a little YouTube quota; existing videos aren't re-fetched).",
+    "loading": "Finding channels…",
+    "empty": "No new channels — every channel in your playlists is one you already follow.",
+    "subscribe": "Subscribe",
+    "subscribeHint": "Subscribe to this channel on YouTube (changes your real account; uses a little API quota). Its videos arrive on the next background sync.",
+    "needWriteHint": "Enable playlist editing in Settings to subscribe on YouTube.",
+    "subscribedTitle": "Subscribed on YouTube",
+    "subscribedBody": "You're now following {{name}} — its new uploads will start arriving in your feed.",
+    "subscribeFailed": "Subscribe failed",
+    "cols": {
+      "inPlaylists": "In playlists",
+      "inPlaylistsHint": "{{videos}} video(s) from this channel across {{playlists}} of your playlist(s)."
+    }
+  },
   "filters": {
     "all": "All",
     "needsFull": "Needs full history",