Fix demo snapshot volume mount (named volume must point to directory not file)

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

demo/docker-compose.yml

@@ -16,10 +16,10 @@ services:
       ALLOW_REGISTRATION: "false"
       BASE_CURRENCY: "GBP"
       DEMO_MODE: "true"
-      DEMO_SNAPSHOT_PATH: "/app/demo_snapshot.sql.gz"
+      DEMO_SNAPSHOT_PATH: "/app/demo_data/demo_snapshot.sql.gz"
     volumes:
       - ../secrets:/run/secrets:ro
-      - demo_snapshot:/app/demo_snapshot.sql.gz:rw
+      - demo_snapshot:/app/demo_data:rw
       - demo_uploads:/app/uploads
     depends_on:
       postgres: