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/alembic/versions/0019_user_yt_channel_id.py

@@ -0,0 +1,28 @@
+"""cache each user's own YouTube channel id
+
+Revision ID: 0019_user_yt_channel_id
+Revises: 0018_maintenance_batch_setting
+Create Date: 2026-06-19
+
+Adds users.yt_channel_id: the user's own YouTube channel id, resolved lazily from
+channels.list?mine=true and cached. Used to exclude the user's own channel from the
+playlist-discovery list (you don't subscribe to yourself). NULL = not resolved yet.
+Purely additive.
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0019_user_yt_channel_id"
+down_revision: Union[str, None] = "0018_maintenance_batch_setting"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column("users", sa.Column("yt_channel_id", sa.String(length=32), nullable=True))
+
+
+def downgrade() -> None:
+    op.drop_column("users", "yt_channel_id")