docs(readme): the music mount is writable — Minstrel deletes when asked

The quickstart mounted the library :ro and promised "Minstrel never
writes to your library". That stopped being true long before #3918:
quarantine's Delete file removes files, and under :ro it failed. The
operator has accepted delete ownership (Scribe note #3926).

The quickstart now mounts it writable and says exactly what Minstrel
writes: it deletes a file when an admin asks, and never moves, renames
or retags. It notes that uid 1000 needs write access, and that :ro
still works, with deletes refusing and explaining why.

Reorganising and tag writes stay out, pending whether Minstrel absorbs
Lidarr's role.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SQ31KQpYbStyK5y58UmPLH

README.md

@@ -37,8 +37,12 @@ services:
     ports: ['4533:4533']
     volumes:
       # Your music library. Point ./music at wherever your audio files
-      # live. Mounted read-only — Minstrel never writes to your library.
+      # live. Writable, because Minstrel deletes a file when an admin asks
-      - ./music:/music:ro
+      # it to (for example, quarantine's "Delete file"). It never moves,
+      # renames or retags anything. The container runs as uid 1000, so that
+      # user needs write access to the folders. Mount it :ro to forbid even
+      # deletes: those actions then refuse, say why, and delete nothing.
+      - ./music:/music
       # Generated data: playlist cover collages, artist art, caches.
       # The path must match MINSTREL_STORAGE_DATA_DIR, which the image
       # sets to /app/data — keep this mount on /app/data or your cache
@@ -47,7 +51,7 @@ services:
     environment:
       MINSTREL_DATABASE_URL: postgres://minstrel:minstrel@db:5432/minstrel?sslmode=disable
       # Colon-separated library roots to scan; must match the container
-      # path of the read-only music mount above (/music here).
+      # path of the music mount above (/music here).
       MINSTREL_LIBRARY_SCAN_PATHS: /music
     depends_on: [db]