ci: 调整 CI 触发分支为 main，运行环境改为 aliyun

.gitea/workflow/backend-ci.yml

@@ -2,15 +2,15 @@ name: Backend CI
 
 on:
   push:
-    branches: [develop, main]
+    branches: [main]
     paths: [backend/**]
   pull_request:
-    branches: [develop, main]
+    branches: [main]
     paths: [backend/**]
 
 jobs:
   ci:
-    runs-on: ubuntu-latest
+    runs-on: aliyun
     defaults:
       run:
         working-directory: backend