Add configurable file/dir perms and UMASK support

Introduce file and directory permission settings (fileChmod, dirChmod) end-to-end. UI: new controls in Paths settings with octal validation and defaults (664/775). API: GET exposes defaults; PUT validates octal strings and upserts configuration keys (file_chmod, dir_chmod) and clears related cache keys. Runtime: read config values in file utilities and services (FileOrganizer, direct-download, chapter-merger, epub-fixer) to apply mkdir modes and chmod files/dirs; FileOrganizer now accepts fileMode/dirMode and getFileOrganizer reads/parses DB settings. Docker: add UMASK option to docker-compose and propagate/apply UMASK in entrypoint/app-start scripts. Tests: update mocks to account for config service usage.

docker/unified/app-start.sh

@@ -22,6 +22,12 @@ PGID=${PGID:-$(id -g node)}
 echo "[App] Starting Next.js server..."
 echo "[App] Process will run as UID:GID = $PUID:$PGID"
 
+# Apply UMASK if set (controls default file permissions)
+if [ -n "$UMASK" ]; then
+    echo "[App] Applying umask: $UMASK"
+    umask "$UMASK"
+fi
+
 cd /app
 
 # =============================================================================