feat(k8s): 新增 core 真应用(前端+Django API+Celery worker)构建与部署

- core/frontend: Vite 多阶段镜像 + nginx 同源反代 /api,/admin,/static(零 CORS)
- core/backend: Django gunicorn 镜像 + entrypoint(自动 migrate/collectstatic)+ WhiteNoise
- k8s/core: api/worker/web Deployment+Service + ingress(airshelf-web.airlabs.art)
- workflow: 追加 core 前后端 build/push,从 core/backend/.env 套生产覆盖生成 env Secret 后部署
- .gitignore 放行 core/backend/.env;.env 白名单加入 airshelf-web 域名
- 含前端 WIP 还原改动

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

k8s/core/ingress.yaml

@@ -0,0 +1,24 @@
+apiVersion: networking.k8s.io/v1
+kind: Ingress
+metadata:
+  name: airshelf-core-ingress
+  annotations:
+    kubernetes.io/ingress.class: "traefik"
+    cert-manager.io/cluster-issuer: "letsencrypt-prod"
+    traefik.ingress.kubernetes.io/router.middlewares: "default-redirect-https@kubernetescrd"
+spec:
+  tls:
+  - hosts:
+    - airshelf-web.airlabs.art
+    secretName: airshelf-core-tls
+  rules:
+  - host: airshelf-web.airlabs.art
+    http:
+      paths:
+      - path: /
+        pathType: Prefix
+        backend:
+          service:
+            name: airshelf-core-web
+            port:
+              number: 80