fix(download): dedup missed re-downloads when the title contains ' - ' (KH triple/quadruple)

findExistingTrack split the filename at the first ' - ' assuming 'Artist - Title',
so a title with internal dashes ('Kingdom Hearts 2 - Dearly Beloved - Main Theme')
or a garbage/absent artist prefix ('01-01 - …') was mis-parsed → the duplicate went
undetected and got re-downloaded (3-4x). Now match the query title against BOTH the
full stem AND the part after the first ' - ', so titles with dashes and prefix-only
filenames are recognized. Normal 'Artist - Title' files still match; no false
positives; remix-vs-base distinction preserved. Unit-tested on the exact KH case.

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

backend/lib/metadata.js

@@ -90,11 +90,18 @@ function findExistingTrack(musicRoot, firstArtist, cleanTitle) {
               if (!AUDIO_EXTS.includes(ext)) continue;
               // Strip extension + leading track prefix ("01 - ", "01-15 – " etc.)
               const stem = f.slice(0, -(ext.length + 1)).replace(/^\d[\d-]*\s*[-–—]\s*/, '');
-              // Stem may be "Artist - Title" or just "Title"
+              // Candidate titles from the filename. The old code only used the part AFTER the first
+              // " - " (assuming "Artist - Title"), which mis-parses titles that themselves contain
+              // " - " (e.g. "Kingdom Hearts 2 - Dearly Beloved - Main Theme") or filenames with a
+              // garbage/absent artist prefix ("01-01 - …") → the duplicate went undetected and got
+              // re-downloaded. Now match if ANY reading equals the query title:
+              //   1) the FULL stem (title with internal dashes, or no clean artist prefix)
+              //   2) the part after the first " - " (normal "Artist - Title")
               const di = stem.indexOf(' - ');
-              const rawTitle = di >= 0 ? stem.slice(di + 3) : stem;
-              const normStemTitle = norm(rawTitle.replace(_RELEASE_SUFFIX_RE, ''));
-              if (titleVariants.has(normStemTitle)) return path.join(albumPath, f);
+              const candidates = di >= 0 ? [stem, stem.slice(di + 3)] : [stem];
+              for (const cand of candidates) {
+                if (titleVariants.has(norm(cand.replace(_RELEASE_SUFFIX_RE, '')))) return path.join(albumPath, f);
+              }
             }
           } catch {}
         }