Fix image export in container and skip already-local URLs

Mount frontend/public into the api container so the export can write
badge/sprite images. Skip re-downloading URLs that are already local
paths from a previous export.

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

docker-compose.yml

@@ -8,6 +8,7 @@ services:
     volumes:
       - ./backend/src:/app/src:cached
       - ./backend/alembic.ini:/app/alembic.ini:cached
+      - ./frontend/public:/frontend/public
     environment:
       - DEBUG=true
       - DATABASE_URL=postgresql://postgres:postgres@db:5432/nuzlocke