Fix GitLab deploy port conflicts on Sinbad2.

Parametrize compose published ports and add CI safeguards to detect and release occupied frontend/backend ports before docker compose up, preventing bind errors on 8075.

docker-compose.yml

@@ -6,13 +6,13 @@ services:
       - ./backend/.env
     environment:
       DATABASE_URL: postgresql+psycopg://genexamenes:genexamenes@db:5432/genexamenes
-      # Sobrescribe backend/.env con el puerto actual del frontend.
-      ALLOWED_ORIGINS: http://sinbad2.ujaen.es,http://sinbad2.ujaen.es:8075
+      # Sobrescribe backend/.env con el origen público del frontend en despliegue.
+      ALLOWED_ORIGINS: ${ALLOWED_ORIGINS:-http://sinbad2.ujaen.es,http://sinbad2.ujaen.es:8075}
       LLM_BASE_URL: 
       LLM_MODEL: qwen3.5:35b
       LLM_TIMEOUT_SECONDS: "180"
     ports:
-      - "8074:8074"
+      - "${BACKEND_PORT:-8074}:8074"
     depends_on:
       db:
         condition: service_healthy
@@ -27,7 +27,7 @@ services:
         VITE_API_URL: ${VITE_API_URL:-http://sinbad2.ujaen.es:8074}
         VITE_GOOGLE_CLIENT_ID: ${VITE_GOOGLE_CLIENT_ID:-}
     ports:
-      - "8075:80"
+      - "${FRONTEND_PORT:-8075}:80"
     depends_on:
       - backend
     restart: unless-stopped