chore(compose): bake in dev mount + scan defaults

Dev stack can now `docker compose up --build` with no extra env: mounts
./music read-only, enables startup scan, and bootstraps admin with a
generated password printed to stderr on first boot.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

docker-compose.yml

@@ -34,8 +34,18 @@ services:
     environment:
       SMARTMUSIC_DATABASE_URL: postgres://minstrel:minstrel@postgres:5432/minstrel?sslmode=disable
       SMARTMUSIC_LOG_FORMAT: text
+      SMARTMUSIC_LIBRARY_SCAN_PATHS: /music
+      SMARTMUSIC_LIBRARY_SCAN_ON_STARTUP: "true"
+      SMARTMUSIC_AUTH_ADMIN_USERNAME: admin
+      # SMARTMUSIC_AUTH_ADMIN_PASSWORD left unset on purpose — bootstrap
+      # generates one and prints it to stderr on first boot. Watch the logs
+      # of the minstrel service the first time you bring the stack up.
     ports:
       - "4533:4533"
+    volumes:
+      # Point ./music at your test library (symlink, bind-mount, whatever).
+      # Read-only so a buggy scanner can't rewrite your files.
+      - ./music:/music:ro
 
 volumes:
   minstrel-pgdata: