fix(backend): remove title keyword filter for Japanese tracks

Japanese Art Tracks on YTM/YouTube are often romanized ("Souiunatsu"
for "そういう夏") — the Japanese keyword filter in --match-filter always
rejected the correct video. Channel + duration are sufficient quality
gates for JP tracks.

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

backend/lib/ytdlp.js

@@ -13,14 +13,10 @@ const { romajiForStorage, isTokenizerReady } = require('./romaji');
 const JP_RE = /[ぁ-んァ-ン一-龯]/;
 
 function buildTitleKeywords(title) {
-  // For Japanese titles: \w strips all kana/kanji, leaving nothing useful.
-  // Instead use the first whitespace-delimited segment as a literal keyword so
-  // the YTM --match-filter requires the video title to contain it.
-  if (JP_RE.test(title)) {
-    const firstWord = (title.split(/[\s　]/)[0] || '').slice(0, 12);
-    const escaped   = firstWord.replace(/[.*+?^${}()|[\]\\]/g, '\\$&');
-    return escaped ? [escaped] : [];
-  }
+  // For Japanese titles: Art Tracks on YTM/YouTube are often romanized
+  // ("Souiunatsu" for "そういう夏") — a Japanese keyword filter would always
+  // reject the correct video. Channel + duration are the quality gates instead.
+  if (JP_RE.test(title)) return [];
   const VARIANT_WORDS = new Set(['sped','speedup','slowed','reverb','nightcore','karaoke','parody','remix','cover','acoustic','instrumental']);
   const STOPWORDS = new Set(['the','a','an','and','or','of','in','on','at','to','for','feat','ft','with','single','ep','album','remaster','remastered','deluxe','edition','version','live','bonus']);
   const MIX_QUALIFIERS = ['radio', 'extended', 'club', 'vip', 'original', 'edit'];