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>

.env.example

@@ -14,3 +14,10 @@ MUSIC_DIR=./music
 # share (SMB/NFS) so temporary downloads don't hammer the share; the final import copies the
 # finished album onto the library volume and swaps it in atomically.
 # STAGING_DIR=/srv/lyra-staging
+
+# Host directory where slskd (the external Soulseek daemon) writes FINISHED downloads,
+# bind-mounted into the worker at /slskd-downloads. The worker moves completed Soulseek
+# transfers out of here into staging, so this MUST be the same directory slskd downloads to.
+# Set this only once Lyra runs on the same host as slskd; until then Soulseek can search but
+# not deliver. Defaults to ./slskd-downloads (an empty local dir) so the stack still starts.
+# SLSKD_DOWNLOADS_DIR=/var/lib/slskd/downloads