Auto-create and chown data dirs on first start

Entrypoint runs as root, ensures /data/{db,uploads,logo} and
/webdav-config exist with UID 1000 ownership, then drops privileges
via gosu. Removes the manual sudo chown step from the README and
makes a fresh docker compose up succeed without prep.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -3,7 +3,8 @@ services:
     build: .
     container_name: simple-file-upload
     restart: unless-stopped
-    user: "1000:1000"
+    # No `user:` here — entrypoint.sh starts as root, chowns the bind mounts
+    # to UID 1000, then drops privileges via gosu before running node.
     ports:
       - "${APP_PORT:-3500}:3000"   # Web (Upload + Admin)
     environment: