feat: normalize Japanese artist/title names to romaji at storage time

- Add romajiForStorage() to romaji.js — smart word spacing + Title Case
- ytdlp.js: apply to folder names, filenames, and embedded tags
- novasonic-server.js: apply in edit-metadata endpoint and Spotify resolve
- spotify.js: update track artist/name from iTunes canonical English names

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

backend/lib/romaji.js

@@ -75,6 +75,25 @@ function toRomaji(text) {
   return _hiraToRomaji(hira).toLowerCase();
 }
 
+// Human-readable romaji for storage: spaces between words, Title Case, ASCII preserved as-is.
+function romajiForStorage(text) {
+  if (!text || !_tokenizer || !JP_RE.test(text)) return text || '';
+  const tokens = _tokenizer.tokenize(text);
+  const parts = tokens.map(t => {
+    const r = t.reading;
+    if (!r || r === '*') return t.surface_form; // ASCII — keep original
+    return _hiraToRomaji(_kataToHira(r));        // Japanese — convert (lowercase)
+  });
+  // Add space between adjacent alphabetic segments (word separation for names/titles)
+  const joined = parts.reduce((acc, part, i) => {
+    if (i === 0) return part;
+    const needsSpace = /[a-zA-Z0-9]$/.test(parts[i - 1]) && /^[a-zA-Z]/.test(part);
+    return acc + (needsSpace ? ' ' : '') + part;
+  }, '');
+  // Title-case: capitalize first lowercase letter of each word, preserve existing uppercase
+  return joined.replace(/\b([a-z])/g, c => c.toUpperCase()).replace(/\s+/g, ' ').trim();
+}
+
 // ─── INDEX BUILDER ────────────────────────────────────────────────────────────
 
 const romajiCache = new Map();
@@ -123,4 +142,4 @@ function clearRomajiCache() { romajiCache.clear(); }
 
 function isTokenizerReady() { return !!_tokenizer; }
 
-module.exports = { buildRomajiIndex, clearRomajiCache, isTokenizerReady, fetchNdSongs: _fetchNdSongs };
+module.exports = { buildRomajiIndex, clearRomajiCache, isTokenizerReady, fetchNdSongs: _fetchNdSongs, romajiForStorage };