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>

CLAUDE.md

@@ -93,6 +93,14 @@ On the deploy host the music lives at `/opt/navidrome/music`.
   matches, download **nothing** and fail cleanly with "Nicht eindeutig gefunden — bitte
   Direktlink im Importer nutzen". Never fall back to a blind title-only/no-filter grab — it
   pulled same-titled covers by other uploaders, and wrong Piano Echoes for Japanese tracks.
+- **Numbers in a title are always required keywords.** `buildTitleKeywords` kept only the two
+  LONGEST words, and numbers are short — so "700 Main Street" was filtered on Street+Main only and
+  the cascade happily saved **"777 Main Street" by a different artist** (step 3 drops the channel
+  check, so nothing else caught it). Numeric tokens are now emitted as `\b<n>\b` in addition to the
+  two longest words. Same fix carries a small abbreviation map (street/st, saint/st, mount/mt,
+  doctor/dr, versus/vs, number/no): uploaders write "700 Main St" where the release says "Street",
+  which had rejected the correct official Art Track in every step. Unit-tested in
+  `backend/test/ytdlp.test.js`.
 - **Size cap:** `--max-filesize 30M` on every yt-dlp call; Mittelweg steps also cap at 2×
   expected duration. Prevents saving a 28-min video (52 MB) when the wrong result slips through.
 - **Leading aesthetic characters:** artists often style their name with leading punctuation