feat: romaji artist names, Apple Music autofill, romaji index invalidation

- Artist grid and detail header now show romaji names when toggle is on.
  Builds a Map<artistId, artistRomaji> from the existing song romaji index
  so no extra API call is needed.

- Apple Music autofill button added to edit modal next to MusicBrainz.
  Reuses the existing lookupItunesTrackMeta backend function via a new
  GET /api/itunes/lookup endpoint — fills album, year, genre, track#,
  and cover art.

- Romaji index is invalidated after every successful download so newly
  added songs appear with correct sort tags without requiring a page reload.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

index.html

@@ -927,6 +927,10 @@
           <svg width="11" height="11" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" style="margin-right:4px"><circle cx="11" cy="11" r="8"/><line x1="21" y1="21" x2="16.65" y2="16.65"/></svg>
           MusicBrainz
         </button>
+        <button class="btn-outline blue" style="font-size:11px;padding:6px 10px;height:fit-content" onclick="autofillItunes()">
+          <svg width="11" height="11" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" style="margin-right:4px"><circle cx="11" cy="11" r="8"/><line x1="21" y1="21" x2="16.65" y2="16.65"/></svg>
+          Apple Music
+        </button>
       </div>
       <div class="info-edit-fields">
         <label class="info-edit-label">Titel<input class="info-edit-input" id="ie-title" type="text" placeholder="Titel"></label>