feat(ci): Runner物理分层 - 构建任务只调度到旧服务器

将6个构建Job（Staging+Production）的runs-on从saas改为[saas, build-farm]，
只有带build-farm标签的旧服务器Runner能接构建任务，新服务器6个Runner
专注跑CI门禁检查，实现真正的物理分层，避免构建任务挤占CI检查资源。

.gitea/workflows/ci-cd.yml

@@ -753,7 +753,7 @@ jobs:
 
   build-staging-api:
     name: Build Staging API Image
-    runs-on: saas
+    runs-on: [saas, build-farm]
     timeout-minutes: 20
     needs: [validate, frontend-lint]
 
@@ -869,7 +869,7 @@ jobs:
 
   build-staging-worker:
     name: Build Staging Worker Image
-    runs-on: saas
+    runs-on: [saas, build-farm]
     timeout-minutes: 20
     needs: [validate, frontend-lint]
 
@@ -985,7 +985,7 @@ jobs:
 
   build-staging-web:
     name: Build Staging Web Image
-    runs-on: saas
+    runs-on: [saas, build-farm]
     timeout-minutes: 20
     needs: [validate, frontend-lint]
 
@@ -1413,7 +1413,7 @@ jobs:
 
   build-production-api:
     name: Build Production API Image
-    runs-on: saas
+    runs-on: [saas, build-farm]
     timeout-minutes: 20
     needs: [validate, frontend-lint]
 
@@ -1510,7 +1510,7 @@ jobs:
 
   build-production-worker:
     name: Build Production Worker Image
-    runs-on: saas
+    runs-on: [saas, build-farm]
     timeout-minutes: 20
     needs: [validate, frontend-lint]
 
@@ -1607,7 +1607,7 @@ jobs:
 
   build-production-web:
     name: Build Production Web Image
-    runs-on: saas
+    runs-on: [saas, build-farm]
     timeout-minutes: 20
     needs: [validate, frontend-lint]