Add support for serving static files from DATA_FILES_DIR

Introduce a new environment variable, `DATA_FILES_DIR`, for configuring static file storage. Updated `docker-compose` files to mount the host directory and propagate the variable. Implemented FastAPI `StaticFiles` to serve files from this directory under the `/files` route.

docker-compose.dev.yml

@@ -26,6 +26,7 @@ services:
       - ./backend:/app
       - ./uploads:/app/uploads
       - backend_dev_modules:/app/.venv
+      - ${HOST_DATA_FILES_DIR}:${DATA_FILES_DIR}
     ports:
       - "8000:8000"
     env_file:
@@ -34,6 +35,8 @@ services:
       - DATABASE_URL=${DATABASE_URL}
       - SECRET_KEY=${SECRET_KEY}
       - ENVIRONMENT=development
+      - DATA_FILES_DIR=${DATA_FILES_DIR}
+      - HOST_DATA_FILES_DIR=${HOST_DATA_FILES_DIR}
       - DEBUG=true
       - BACKEND_CORS_ORIGINS=${BACKEND_CORS_ORIGINS}
     depends_on: