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/test/metadata.test.js

@@ -0,0 +1,50 @@
+const { test, before, after } = require('node:test');
+const assert = require('node:assert/strict');
+const fs = require('fs');
+const os = require('os');
+const path = require('path');
+const { findExistingTrack } = require('../lib/metadata');
+
+let ROOT;
+const touch = (artist, album, file) => {
+  const dir = path.join(ROOT, artist, album);
+  fs.mkdirSync(dir, { recursive: true });
+  fs.writeFileSync(path.join(dir, file), '');
+};
+
+before(() => {
+  ROOT = fs.mkdtempSync(path.join(os.tmpdir(), 'ns-dedup-'));
+  // KH single: normal "01 - " prefix, title contains multiple " - "
+  touch('Geek Music', 'Kingdom Hearts 2 - Dearly Beloved - Main Theme - Single',
+        '01 - Kingdom Hearts 2 - Dearly Beloved - Main Theme.mp3');
+  // KH album variant: garbage "01-01" artist prefix, no track number
+  touch('Geek Music', 'Kingdom Hearts 2 - Dearly Beloved - Main Theme',
+        '01-01 - Kingdom Hearts 2 - Dearly Beloved - Main Theme.mp3');
+  // Normal "Artist - Title" file
+  touch('Some Artist', 'Therapy - Single', '01 - Some Artist - Therapy.mp3');
+});
+after(() => { try { fs.rmSync(ROOT, { recursive: true, force: true }); } catch {} });
+
+test('detects a re-download when the title itself contains " - " (KH bug)', () => {
+  assert.ok(findExistingTrack(ROOT, 'Geek Music', 'Kingdom Hearts 2 - Dearly Beloved - Main Theme'));
+});
+
+test('detects it even with a garbage "01-01 - " artist prefix', () => {
+  // Both KH files should resolve; at least one must be found → dedup skips the re-download.
+  const hit = findExistingTrack(ROOT, 'Geek Music', 'Kingdom Hearts 2 - Dearly Beloved - Main Theme');
+  assert.ok(hit && hit.includes('Kingdom Hearts 2 - Dearly Beloved - Main Theme'));
+});
+
+test('normal "Artist - Title" filename still matches on title alone', () => {
+  assert.ok(findExistingTrack(ROOT, 'Some Artist', 'Therapy'));
+});
+
+test('no false positive: a different title is not matched', () => {
+  assert.equal(findExistingTrack(ROOT, 'Geek Music', 'Some Unrelated Song'), null);
+  assert.equal(findExistingTrack(ROOT, 'Some Artist', 'Kingdom Hearts 2'), null);
+});
+
+test('remix stays distinct from the base track (no regression from the version-marker fix)', () => {
+  assert.equal(findExistingTrack(ROOT, 'Some Artist', 'Therapy (Remix)'), null);
+  assert.ok(findExistingTrack(ROOT, 'Some Artist', 'Therapy')); // base still found
+});