Use named volume for downloads so DOWNLOAD_PATH from stack.env is respected on every redeploy

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -6,7 +6,7 @@ services:
     restart: unless-stopped
     volumes:
       - ./data/db:/data
-      - ${DOWNLOAD_PATH:-./downloads}:/downloads
+      - downloads:/downloads
     environment:
       DATABASE_URL: sqlite:////data/app.db
       DOWNLOAD_PATH: /downloads
@@ -21,4 +21,10 @@ services:
     depends_on:
       - backend
 
-volumes: {}
+volumes:
+  downloads:
+    driver: local
+    driver_opts:
+      type: none
+      o: bind
+      device: ${DOWNLOAD_PATH:-/tmp/ythub-downloads}