fix: force IPv4 in worker, count real Qobuz files, flatten nested output

- worker had no IPv6 route -> Qobuz CDN (dual-stack) failed ~half the tracks
  with 'Network is unreachable'; disable IPv6 in the container (IPv4 only).
- StreamripClient counted metadata tracks (always full) -> a partial download
  falsely imported; now count the actual files written (short count -> needs_attention).
- streamrip writes into a nested 'Artist - Album [..]/' folder the tagger never saw;
  flatten audio files up into the album dir so tagging/organization runs.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -35,6 +35,10 @@ services:
     environment:
       DATABASE_URL: ${DATABASE_URL}
       LYRA_SECRET_KEY: ${LYRA_SECRET_KEY}
+    # Qobuz's CDN also resolves to IPv6, but the container has no IPv6 route
+    # ("Network is unreachable"); disable IPv6 so downloads use IPv4 only.
+    sysctls:
+      - net.ipv6.conf.all.disable_ipv6=1
     volumes:
       - ${MUSIC_DIR:-./music}:/music
     depends_on: