fix(compose): mount ./secrets read-write so yt-dlp can save cookie jar

yt-dlp calls save_cookies() on exit and rewrites the --cookies file. A read-only
mount makes it crash with "OSError: [Errno 30] Read-only file system", which surfaced
as 500s on the app's YouTube search/download endpoints when YT_COOKIE_FILE was set.

docker-compose.yml

@@ -24,8 +24,10 @@ services:
     volumes:
       - backend-data:/app/packages/backend/data
       - music-data:/data/music
-      # Mounted read-only; the actual cookie file is gitignored (never committed).
-      - ./secrets:/secrets:ro
+      # Read-write on purpose: yt-dlp rewrites the cookie jar on exit (save_cookies),
+      # so a :ro mount makes it crash with "Read-only file system". The actual cookie
+      # file is gitignored (never committed).
+      - ./secrets:/secrets
     ports:
       - "3001:3001"
     depends_on: