Add optional bookdrop volume and .gitignore entry

Document an optional 'bookdrop' host folder in docker-compose.yml with a commented example volume mount for the Manual Import (Admin → audiobook → Manual Import) file picker, and add /bookdrop to .gitignore so local bookdrop mounts are not tracked.

.gitignore

@@ -53,4 +53,5 @@ next-env.d.ts
 /redis
 /pgdata
 /test-media
-/test-data
+/test-data
+/bookdrop

docker-compose.yml

@@ -17,6 +17,11 @@ services:
       - ./downloads:/downloads
       - ./media:/media
 
+      # Book Drop: optional folder for Manual Import (Admin → audiobook → Manual Import)
+      # Map any host folder here and it will appear as a browsable root in the file picker.
+      # Example: - /path/to/your/audiobooks:/bookdrop
+      # - ./bookdrop:/bookdrop
+
       # PostgreSQL data persistence
       - ./pgdata:/var/lib/postgresql/data