feat: ElevenLabs-Voice-Liste + Fehlerdetails in Audio-Batch-Ergebnissen

- GET /api/tts-settings/voices/available listet die Account-Stimmen
  (Grundlage für Voice-Auswahl im CMT statt Freitext-IDs)
- Audio-Batch/-Fill-Fehler enthalten jetzt das ElevenLabs-Detail
  (z.B. voice_not_found) statt nur 'ElevenLabs error'

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

src/lib/pipeline.js

@@ -5,7 +5,7 @@ const { query } = require('../db');
 const { LANGS, fillMissingRow } = require('./translate');
 const { translateWordGroup } = require('./pairContent');
 const { generatePairsForObject, persistPair } = require('./generatePairs');
-const { generateAndStore } = require('../routes/audios');
+const { generateAndStore, describeError } = require('../routes/audios');
 
 const queue = [];
 let running = false;
@@ -167,7 +167,7 @@ async function runPicture(pictureId) {
         await generateWithBackoff(u);
         progress.audiosDone++;
       } catch (err) {
-        failures.push(`${u.source_table}/${u.source_field}/${u.language}: ${err.message}`);
+        failures.push(`${u.source_table}/${u.source_field}/${u.language}: ${describeError(err)}`);
       }
       await setStep(pictureId, 'audio', progress);
     }