feat(radio): wire radio + similarity into the queue (A11/B6, §6.5)

"Start radio" in the track menu seeds the queue from a track; RadioController
(headless, in AppShell) tops it up via /radio/next as it drains. QueuePanel's
radio scaffolding is now live: the exploration slider drives familiar<->new and
each entry shows its reason ("why this is playing"). Adds the radio endpoints
(start/next/similar), radio context + actions on the queue slice, and the pick->
entry mapper. Degrades to metadata picks while ML is offline.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/api/endpoints/radio.ts

@@ -0,0 +1,67 @@
+import { api } from '../index';
+import { toTrack, type RawTrack } from '../mappers';
+import type { RadioResult } from '../types';
+
+// Radio + similarity (§6.5). Stateless: the client passes the seed + already-
+// queued ids and pulls more as the queue drains.
+
+interface RawRadioTrack {
+  track: RawTrack;
+  reason: string;
+}
+interface RawRadio {
+  source: string;
+  tracks: RawRadioTrack[];
+}
+interface RawSimilar {
+  source: string;
+  tracks: RawTrack[];
+}
+
+export interface RadioArgs {
+  seedTrackId?: string;
+  fromLikes?: boolean;
+  exploration?: number;
+  count?: number;
+  excludeIds?: string[];
+}
+
+const toBody = (a: RadioArgs) => ({
+  seed_track_id: a.seedTrackId ?? null,
+  from_likes: a.fromLikes ?? false,
+  exploration: a.exploration ?? 0.25,
+  count: a.count ?? 20,
+  exclude_ids: a.excludeIds ?? [],
+});
+
+const toRadio = (r: RawRadio): RadioResult => ({
+  source: r.source,
+  picks: r.tracks.map((x) => ({ track: toTrack(x.track), reason: x.reason })),
+});
+
+export const radioApi = api.injectEndpoints({
+  endpoints: (build) => ({
+    startRadio: build.mutation<RadioResult, RadioArgs>({
+      query: (a) => ({ url: '/radio', method: 'POST', body: toBody(a) }),
+      transformResponse: (r: RawRadio) => toRadio(r),
+    }),
+    radioNext: build.mutation<RadioResult, RadioArgs>({
+      query: (a) => ({ url: '/radio/next', method: 'POST', body: toBody(a) }),
+      transformResponse: (r: RawRadio) => toRadio(r),
+    }),
+    getSimilarTracks: build.query<RadioResult, string>({
+      query: (trackId) => `/tracks/${trackId}/similar`,
+      transformResponse: (r: RawSimilar) => ({
+        source: r.source,
+        picks: r.tracks.map((t) => ({ track: toTrack(t), reason: 'similar' })),
+      }),
+    }),
+  }),
+  overrideExisting: false,
+});
+
+export const {
+  useStartRadioMutation,
+  useRadioNextMutation,
+  useGetSimilarTracksQuery,
+} = radioApi;