fix: use the US iTunes store everywhere for consistent artist naming

Search (web api.js) was JP-first, the download lookups were JP-first, and the Android
client was already US — so the same artist resolved inconsistently: sometimes kanji,
sometimes katakana, sometimes romaji, depending on which path and which store matched.

Switch everything to the US store:
- api.js itunesSearch → country=us (one fetch, matches the Android client)
- metadata.js lookupItunesTrackMeta → US only (drops the JP fetch: consistent names +
  halves the download's iTunes calls, which also eases the rate limit)
- searchItunesAlbumTracks → country=us
- remove the JP-katakana guards in the album/artist download paths (redundant now, and
  they wrongly romanized genuine Japanese artists like 羽岡佳 → "Kei Haneoka")

Titles still come from the source, so a Japanese title the user picked stays Japanese;
only iTunes-derived fields (album_artist, genre, year, artwork) follow the US store.

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

backend/lib/metadata.js

@@ -168,24 +168,18 @@ async function lookupItunesTrackMeta(artist, title) {
     );
     const titleOnlyMatch = results => results.find(t => normalizeTitle(t.trackName) === normT);
 
-    // Fetch both in parallel — JP for correct Japanese titles/artwork, US for English genre names
-    const [jpResults, usResults] = await Promise.all([fetchItunes('jp'), fetchItunes('us')]);
-    let match = strictMatch(jpResults);
-    // titleOnlyMatch (no artist check) is too permissive for Japanese titles/artists —
-    // common anime/OST titles exist on both JP and US stores under different artists.
-    // Only use it as a fallback when neither title nor artist contains Japanese characters.
+    // US store only — one consistent naming source (English/romaji) across search + download, so the
+    // same artist always resolves the same way instead of "sometimes Japanese, sometimes romaji". A
+    // single request instead of JP+US also halves the download's iTunes calls (helps the rate limit).
+    const usResults = await fetchItunes('us');
     const JP_RE = /[぀-ヿ㐀-䶿一-鿿]/;
     const isJpSearch = JP_RE.test(normT) || JP_RE.test(normA);
-    let usMatch = strictMatch(usResults) || (!isJpSearch && titleOnlyMatch(usResults));
-    if (!match) match = usMatch;
+    // titleOnlyMatch (no artist check) is too permissive when the title is Japanese — common anime/
+    // OST titles exist under different artists — so only use it as a fallback for non-Japanese titles.
+    const match = strictMatch(usResults) || (!isJpSearch && titleOnlyMatch(usResults));
     if (!match) return {};
-    // For non-Japanese songs, take the NAMES from the US store: the JP store returns katakana-
-    // localized artist / album-artist names for Western artists (e.g. "DJスネイク" for DJ Snake),
-    // which would otherwise become the album_artist tag and the on-disk folder. Japanese songs keep
-    // the JP names (correct kanji). Numeric fields (year, track#, duration) are store-agnostic.
-    const naming = (!isJpSearch && usMatch) ? usMatch : match;
-    // JP genre names are localized (e.g. イージーリスニング); prefer US genre, map JP→EN as fallback
-    const rawGenre = (usMatch || match).primaryGenreName;
+    const naming = match;
+    const rawGenre = match.primaryGenreName;
     const genre = rawGenre ? toEnglishGenre(rawGenre) : undefined;
     return {
       album:            naming.collectionName       || '',
@@ -225,7 +219,7 @@ async function fetchItunesTracks(collectionId) {
 async function searchItunesAlbumTracks(artist, album) {
   return new Promise(resolve => {
     const q = encodeURIComponent(`${artist} ${album}`);
-    https.get(`https://itunes.apple.com/search?term=${q}&media=music&entity=album&limit=5&country=jp&explicit=Yes`, res => {
+    https.get(`https://itunes.apple.com/search?term=${q}&media=music&entity=album&limit=5&country=us&explicit=Yes`, res => {
       let d = '';
       res.on('data', c => d += c);
       res.on('end', () => {