feat(metadata): implement single-track metadata editor page (§A7)

Replace the placeholder with a controlled form for title/artist/album/
year/genre/track number, an AcoustID "find matches" action showing
ranked candidates with confidence, a diff/apply picker, a re-enrich
button, and save via PUT /metadata. Adds matches/apply API endpoints,
mappers, types, and en/ru i18n strings. Batch editor remains a
placeholder (deferred).

src/api/endpoints/library.ts

@@ -2,10 +2,12 @@ import { api } from '../index';
 import {
   toAlbum,
   toArtist,
+  toMetadataMatch,
   toPage,
   toTrack,
   type RawAlbum,
   type RawArtist,
+  type RawMetadataMatch,
   type RawPaged,
   type RawTrack,
 } from '../mappers';
@@ -13,6 +15,8 @@ import type {
   Track,
   Album,
   Artist,
+  MetadataEdit,
+  MetadataMatch,
   PaginatedResponse,
   LibraryFilters,
 } from '../types';
@@ -161,6 +165,41 @@ export const libraryApi = api.injectEndpoints({
       }),
       providesTags: ['Track', 'Album', 'Artist'],
     }),
+    getMetadataMatches: build.query<MetadataMatch[], string>({
+      query: (trackId) => `/tracks/${trackId}/metadata/matches`,
+      transformResponse: (raw: { items: RawMetadataMatch[] }) =>
+        raw.items.map(toMetadataMatch),
+    }),
+    applyMetadata: build.mutation<
+      Track,
+      { trackId: string; edit: MetadataEdit }
+    >({
+      query: ({ trackId, edit }) => ({
+        url: `/tracks/${trackId}/metadata`,
+        method: 'PUT',
+        body: {
+          title: edit.title,
+          artist_name: edit.artistName,
+          album_title: edit.albumTitle,
+          year: edit.year,
+          genre: edit.genre,
+          track_number: edit.trackNumber,
+        },
+      }),
+      transformResponse: (raw: RawTrack) => toTrack(raw),
+      invalidatesTags: (_r, _e, { trackId }) => [
+        { type: 'Track', id: trackId },
+        'Album',
+        'Artist',
+      ],
+    }),
+    enrichTrack: build.mutation<{ track_id: string; job_id: string }, string>({
+      query: (trackId) => ({
+        url: `/tracks/${trackId}/metadata/enrich`,
+        method: 'POST',
+      }),
+      invalidatesTags: (_r, _e, trackId) => [{ type: 'Track', id: trackId }],
+    }),
   }),
   overrideExisting: false,
 });
@@ -176,4 +215,7 @@ export const {
   useGetArtistAlbumsQuery,
   useGetArtistTracksQuery,
   useSearchLibraryQuery,
+  useLazyGetMetadataMatchesQuery,
+  useApplyMetadataMutation,
+  useEnrichTrackMutation,
 } = libraryApi;