feat(backend): Mittelweg cascade — never download wrong song on no duration match

When a track's duration is known but nothing matches the duration window, the
cascade no longer falls back to blind title-only/no-filter grabs (which pulled
same-titled videos by other uploaders, e.g. a "recognize" fan clip). Instead it
relaxes only the duration constraint while still REQUIRING the artist channel +
title. If nothing matches, the download produces no file and the job now fails
cleanly with "Nicht eindeutig gefunden — bitte Direktlink im Importer nutzen"
(previously it falsely reported success with no file).

Single, album and playlist paths all surface the not-found state.

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

backend/novasonic-server.js

@@ -601,6 +601,8 @@ app.post('/api/download', async (req, res) => {
         const result = await downloadTrack({ jobId, artist: finalArtist, rawArtist: artist, title, ...meta, sourceUrl: sourceUrl || sp.sourceUrl || undefined });
         if (result?.skipped) {
           updateJob(jobId, { status: 'done', progress: 100, message: 'Bereits vorhanden – übersprungen', skipped: true });
+        } else if (result?.notFound) {
+          updateJob(jobId, { status: 'error', message: 'Nicht eindeutig gefunden — bitte Direktlink im Importer nutzen' });
         } else {
           updateJob(jobId, { status: 'done', progress: 100, message: 'Fertig!' });
         }
@@ -653,7 +655,7 @@ app.post('/api/download', async (req, res) => {
         const plTracks = req.body.tracks || [];
         if (!plTracks.length) throw new Error('Keine Tracks übergeben');
         log(jobId, `🎵 Playlist "${title}" — ${plTracks.length} Tracks`);
-        let done = 0, skipped = 0;
+        let done = 0, skipped = 0, notFound = 0;
         const CONC = CONFIG.maxParallel || 2;
         for (let i = 0; i < plTracks.length; i += CONC) {
           const batch = plTracks.slice(i, i + CONC);
@@ -709,10 +711,13 @@ app.post('/api/download', async (req, res) => {
               log(jobId, `⚠️  Fehler bei "${batch[j].trackName || '?'}": ${r.reason?.message || r.reason}`);
             } else if (r.value?.skipped) {
               skipped++;
+            } else if (r.value?.notFound) {
+              notFound++;
+              log(jobId, `⚠️  Nicht eindeutig gefunden: "${batch[j].trackName || '?'}" — Direktlink nutzen`);
             } else {
               done++;
             }
-            updateJob(jobId, { progress: Math.round(((done + skipped) / plTracks.length) * 90) });
+            updateJob(jobId, { progress: Math.round(((done + skipped + notFound) / plTracks.length) * 90) });
           }
         }
         const allSkipped = skipped === plTracks.length;
@@ -720,7 +725,7 @@ app.post('/api/download', async (req, res) => {
           status: 'done', progress: 100, skipped: allSkipped,
           message: allSkipped
             ? `Alle ${skipped} Tracks bereits vorhanden`
-            : `Fertig! ${done} geladen${skipped ? `, ${skipped} übersprungen` : ''}`,
+            : `Fertig! ${done} geladen${skipped ? `, ${skipped} übersprungen` : ''}${notFound ? `, ${notFound} nicht gefunden` : ''}`,
         });
       }
     } catch (err) {