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/components/Channels.tsx

@@ -20,9 +20,12 @@ import { notify } from "../lib/notifications";
 import Tooltip from "./Tooltip";
 import Avatar from "./Avatar";
 import DataTable, { type Column } from "./DataTable";
+import ChannelDiscovery from "./ChannelDiscovery";
 import TagManager from "./TagManager";
 import { useConfirm } from "./ConfirmProvider";
 
+type ChannelsView = "subscribed" | "discovery";
+
 export type ChannelStatusFilter = "all" | "needs_full" | "fully_synced" | "hidden";
 
 // Compact total-duration label for a whole channel (hours-scale, so H:MM:SS would be huge).
@@ -64,6 +67,17 @@ export default function Channels({
   const qc = useQueryClient();
   const confirm = useConfirm();
 
+  // Which tab is showing: the user's subscriptions, or channels discovered from their
+  // playlists. Persisted so a reload keeps the active tab (see other siftlode.* keys).
+  const [view, setView] = useState<ChannelsView>(() =>
+    localStorage.getItem("siftlode.channelsView") === "discovery"
+      ? "discovery"
+      : "subscribed"
+  );
+  useEffect(() => {
+    localStorage.setItem("siftlode.channelsView", view);
+  }, [view]);
+
   // A YouTube-gated action (sync, backfill, unsubscribe) that 403s means the user hasn't
   // granted the needed scope — surface that with a "Connect" action instead of a vague fail.
   const notifyActionError = (err: unknown, fallbackKey: string) => {
@@ -348,8 +362,39 @@ export default function Channels({
     </div>
   );
 
+  const tabs = (
+    <div className="px-4 pt-4 max-w-7xl mx-auto">
+      <div className="flex flex-wrap items-center gap-1.5">
+        {(["subscribed", "discovery"] as ChannelsView[]).map((v) => (
+          <button
+            key={v}
+            onClick={() => setView(v)}
+            className={`text-sm px-3 py-1.5 rounded-full border transition ${
+              view === v
+                ? "bg-accent text-accent-fg border-accent"
+                : "bg-card border-border text-muted hover:border-accent"
+            }`}
+          >
+            {t(`channels.tabs.${v}`)}
+          </button>
+        ))}
+      </div>
+    </div>
+  );
+
+  if (view === "discovery") {
+    return (
+      <>
+        {tabs}
+        <ChannelDiscovery canWrite={canWrite} onOpenWizard={onOpenWizard} />
+      </>
+    );
+  }
+
   return (
-    <div className="p-4 max-w-7xl mx-auto">
+    <>
+    {tabs}
+    <div className="px-4 pb-4 pt-3 max-w-7xl mx-auto">
       {/* Per-user sync status + catalog-wide actions on one row (search/tags filtering
           now lives in the table headers). */}
       <div className="flex items-start justify-between gap-4 flex-wrap mb-4">
@@ -468,6 +513,7 @@ export default function Channels({
         />
       )}
     </div>
+    </>
   );
 }