fix(worker): retrieve Soulseek downloads from slskd's dir into staging

SlskdClient enqueued the transfer and polled to Completed but never moved any
files into `dest` — slskd saves them to its OWN downloads dir, so staging stayed
empty and every Soulseek job failed "incomplete download". Qobuz/YouTube write
straight into staging and were unaffected.

- SlskdClient._retrieve() moves the finished files out of slskd's downloads dir
  into staging after completion, matching tolerantly by basename and preferring
  a parent-folder + size match (slskd's on-disk layout varies by version).
- download() returns the moved count as track_count so the pipeline's
  completeness check reflects what actually landed; raises loudly if nothing was
  found (misconfigured/unmounted downloads dir).
- Downloads dir resolves slskd.downloadsDir Config > SLSKD_DOWNLOADS_ROOT env >
  /slskd-downloads default; docker-compose mounts ${SLSKD_DOWNLOADS_DIR} there.
- .env.example documents SLSKD_DOWNLOADS_DIR; retrieval unit-tested offline.

slskd runs as an external daemon, so the shared dir must be a host path both
slskd and the worker mount — set once Lyra runs on the slskd host. Not yet
verified live end-to-end (no slskd access from this VM).

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

docker-compose.yml

@@ -36,6 +36,7 @@ services:
       DATABASE_URL: ${DATABASE_URL}
       LYRA_SECRET_KEY: ${LYRA_SECRET_KEY}
       STAGING_ROOT: /staging
+      SLSKD_DOWNLOADS_ROOT: /slskd-downloads
     # 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:
@@ -45,6 +46,13 @@ services:
       # Per-job download staging on its own volume. Defaults to MUSIC_DIR/.staging (prior
       # behavior); set STAGING_DIR to a fast local path when MUSIC_DIR is a network share.
       - ${STAGING_DIR:-${MUSIC_DIR:-./music}/.staging}:/staging
+      # slskd (an EXTERNAL daemon, not in this compose stack) writes finished Soulseek
+      # downloads into its own directory; the worker moves them out of here into staging.
+      # Point SLSKD_DOWNLOADS_DIR at slskd's downloads dir on the host (must be the SAME
+      # path slskd writes to — set it once Lyra runs on the slskd host). Defaults to an
+      # empty local dir so the mount is always valid; Soulseek delivery only works once
+      # this points at the real slskd downloads directory.
+      - ${SLSKD_DOWNLOADS_DIR:-./slskd-downloads}:/slskd-downloads
     depends_on:
       db:
         condition: service_healthy