fix(backend): Zahlen im Titel sind Pflicht-Keywords, Abkuerzungen erlaubt

"700 Main Street" lud "777 Main Street" eines fremden Artists: buildTitleKeywords
behielt nur die zwei laengsten Woerter (Street, Main), die unterscheidende 700
fiel als zu kurz raus — und Schritt 3 der Kaskade prueft den Kuenstler-Channel
nicht mehr. Zahlen werden jetzt immer als \b<n>\b gefordert. Dazu eine kleine
Abkuerzungstabelle (street/st, saint/st, mount/mt, doctor/dr, versus/vs,
number/no), denn der korrekte Art Track heisst "700 Main st" und scheiterte
bisher an jedem Schritt. Tests in backend/test/ytdlp.test.js.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/lib/ytdlp.js

@@ -26,9 +26,23 @@ function buildTitleKeywords(title) {
     .replace(/[^\w\s]/g, ' ')
     .split(/\s+/)
     .filter(w => w.length >= 3 && !VARIANT_WORDS.has(w.toLowerCase()) && !STOPWORDS.has(w.toLowerCase()));
-  const top2 = words.sort((a, b) => b.length - a.length).slice(0, 2);
-  const toPattern = w => (w.length >= 4 && w.endsWith('s')) ? w + '?' : w;
-  const result = top2.map(toPattern);
+  // Numbers are the most distinctive token a title has and they are usually SHORT, so the
+  // longest-two rule threw them away: "700 Main Street" kept only Street+Main and happily matched
+  // "777 Main Street" by a different artist. Numbers are therefore always required, on top of the
+  // two longest words.
+  const numbers = words.filter(w => /^\d+$/.test(w));
+  const wordsNoNum = words.filter(w => !/^\d+$/.test(w));
+  const top2 = wordsNoNum.sort((a, b) => b.length - a.length).slice(0, 2);
+  // Uploaders abbreviate the same word the release spells out ("700 Main St" vs "700 Main Street"),
+  // which rejected the official Art Track in every cascade step. Accept either spelling; \b keeps
+  // the short form from matching inside another word.
+  const ABBREV = { street: 'st', saint: 'st', mount: 'mt', doctor: 'dr', versus: 'vs', number: 'no' };
+  const toPattern = w => {
+    const abbr = ABBREV[w.toLowerCase()];
+    if (abbr) return `\\b(?:${w}|${abbr})\\b`;
+    return (w.length >= 4 && w.endsWith('s')) ? w + '?' : w;
+  };
+  const result = [...numbers.map(n => `\\b${n}\\b`), ...top2.map(toPattern)];
   const qualifier = lower.includes('mix') ? MIX_QUALIFIERS.find(q => lower.includes(q)) : null;
   if (qualifier && !top2.some(w => w.toLowerCase() === qualifier)) result.push(qualifier);
   return result;
@@ -424,4 +438,4 @@ async function embedMetadata({ jobId, audioFile, dir, trackPrefix, safeTitle, ra
   return { success: true, dir };
 }
 
-module.exports = { downloadTrack, embedMetadata };
+module.exports = { downloadTrack, embedMetadata, buildTitleKeywords };