feat: add dev Dockerfile with VNC/X11/Xvfb and update docker-compose.dev.yml

- Create deploy/docker/backend-dev.Dockerfile: single-stage, root,
  includes xauth, x11vnc, novnc, websockify, xvfb, Playwright+chromium,
  Python3+pip+ocr-worker (--break-system-packages)
- Use HTTP (not HTTPS) for apt mirrors per BuildKit TLS note
- Update docker-compose.dev.yml backend to point to backend-dev.Dockerfile
- .dockerignore unchanged (no issues found)

docker-compose.dev.yml

@@ -10,7 +10,11 @@ services:
     ports:
       - "${POSTGRES_PORT:-5432}:5432"
     healthcheck:
-      test: ["CMD-SHELL", "pg_isready -U ${POSTGRES_USER:-postgres} -d ${POSTGRES_DB:-order_site}"]
+      test:
+        [
+          "CMD-SHELL",
+          "pg_isready -U ${POSTGRES_USER:-postgres} -d ${POSTGRES_DB:-order_site}",
+        ]
       interval: 5s
       timeout: 5s
       retries: 20
@@ -20,7 +24,7 @@ services:
   backend:
     build:
       context: .
-      dockerfile: deploy/docker/backend.Dockerfile
+      dockerfile: deploy/docker/backend-dev.Dockerfile
     restart: unless-stopped
     env_file:
       - .env
@@ -30,7 +34,8 @@ services:
       postgres:
         condition: service_healthy
     healthcheck:
-      test: ["CMD-SHELL", "curl -fsS http://127.0.0.1:3000/health/ready || exit 1"]
+      test:
+        ["CMD-SHELL", "curl -fsS http://127.0.0.1:3000/health/ready || exit 1"]
       interval: 5s
       timeout: 5s
       retries: 20