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.

backend/app/youtube/client.py

@@ -168,6 +168,15 @@ class YouTubeClient:
             if not page_token:
                 break
 
+    def get_my_channel_id(self) -> str | None:
+        """The authenticated user's own channel id (channels.list?mine=true). OAuth only;
+        1 quota unit. Returns None if the account has no channel."""
+        data = self._get(
+            "channels", {"part": "id", "mine": "true", "maxResults": 1}, allow_key=False
+        )
+        items = data.get("items", [])
+        return items[0].get("id") if items else None
+
     def get_channels(self, channel_ids: list[str]) -> list[dict]:
         items: list[dict] = []
         for batch in _chunks(channel_ids, 50):
@@ -213,6 +222,23 @@ class YouTubeClient:
                 f"DELETE subscriptions -> {resp.status_code}: {resp.text[:300]}"
             )
 
+    def insert_subscription(self, channel_id: str) -> str:
+        """Subscribe to a channel on YouTube. Requires the write scope and the user's
+        OAuth token (never the public API key). subscriptions.insert costs 50 quota
+        units. Returns the new subscription resource id — store it as
+        `yt_subscription_id` so the channel can later be unsubscribed."""
+        data = self._write(
+            "POST",
+            "subscriptions",
+            params={"part": "snippet"},
+            json={
+                "snippet": {
+                    "resourceId": {"kind": "youtube#channel", "channelId": channel_id}
+                }
+            },
+        )
+        return data.get("id", "")
+
     def iter_playlist_items_with_ids(self, playlist_id: str) -> Iterator[dict]:
         """Yield {item_id, video_id} for each item of one of the user's playlists, in
         playlist order. `item_id` is the playlistItem resource id, needed to delete or