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.yml

@@ -26,6 +26,8 @@ services:
     environment:
       - DATABASE_URL=${DATABASE_URL}
       - SECRET_KEY=${SECRET_KEY}
+      - DATA_FILES_DIR=${DATA_FILES_DIR}
+      - HOST_DATA_FILES_DIR=${HOST_DATA_FILES_DIR}
       - ENVIRONMENT=production
       - DEBUG=false
       - BACKEND_CORS_ORIGINS=${BACKEND_CORS_ORIGINS}
@@ -35,6 +37,8 @@ services:
     networks:
       - app-network
     restart: unless-stopped
+    volumes:
+      - ${HOST_DATA_FILES_DIR}:${DATA_FILES_DIR}
 
   frontend:
     build: