fix(ci): 彻底修复migration diff step卡住问题 — 用后台进程+轮询替代timeout命令

- Gitea Actions v0.2.6不支持step级continue-on-error和timeout-minutes
- timeout命令在host runner环境下行为不可靠（可能不生效）
- 改用后台进程+30秒轮询超时，确保无论fetch成功/失败/卡住都在30秒内返回
- set +e + exit 0确保step永远不会失败，真正做到准备工作不阻断主流程

.gitea/workflows/ci-cd.yml

@@ -143,13 +143,11 @@ jobs:
 
         '
     - name: Prepare git for migration diff
-      continue-on-error: true
-      timeout-minutes: 2
       shell: sh
       env:
         GITHUB_TOKEN: ${{ github.token }}
         GITHUB_SERVER_URL: ${{ github.server_url }}
-      run: "set -eu\nif command -v git >/dev/null 2>&1; then\n  if [ ! -d .git ]; then\n    git init -q\n    git config user.email \"ci@localhost\"\n    git config user.name \"CI\"\n    git add .\n    git commit -q -m \"current\"\n    REPO_URL=\"https://x-access-token:${GITHUB_TOKEN}@${GITHUB_SERVER_URL#https://}/${GITHUB_REPOSITORY}.git\"\n    git remote add origin \"$REPO_URL\"\n  fi\n  timeout 30 git fetch origin develop --depth=1 -q 2>/dev/null || echo \"WARN: cannot fetch develop, will check all migrations\"\nelse\n  echo \"WARN: git not available, will check all migrations\"\nfi\n"
+      run: "set +e\nif command -v git >/dev/null 2>&1; then\n  if [ ! -d .git ]; then\n    git init -q\n    git config user.email \"ci@localhost\"\n    git config user.name \"CI\"\n    git add .\n    git commit -q -m \"current\"\n    REPO_URL=\"https://x-access-token:${GITHUB_TOKEN}@${GITHUB_SERVER_URL#https://}/${GITHUB_REPOSITORY}.git\"\n    git remote add origin \"$REPO_URL\" 2>/dev/null\n  fi\n  # 后台fetch + 30秒超时保护，防止git fetch卡住拖慢CI\n  git fetch origin develop --depth=1 -q >/dev/null 2>&1 &\n  FETCH_PID=$!\n  for i in $(seq 1 30); do\n    if ! kill -0 $FETCH_PID 2>/dev/null; then\n      break\n    fi\n    sleep 1\n  done\n  if kill -0 $FETCH_PID 2>/dev/null; then\n    kill $FETCH_PID 2>/dev/null\n    echo \"WARN: git fetch timeout after 30s, will check all migrations\"\n  else\n    wait $FETCH_PID 2>/dev/null\n    if [ $? -eq 0 ]; then\n      echo \"INFO: fetched origin/develop successfully\"\n    else\n      echo \"WARN: git fetch failed, will check all migrations\"\n    fi\n  fi\nelse\n  echo \"WARN: git not available, will check all migrations\"\nfi\nexit 0\n"
     - name: Check migration safety
       shell: sh
       run: "set -eu\nif git rev-parse origin/develop >/dev/null 2>&1; then\n  python3 scripts/check_migration_safety.py --allow-medium-risk --diff-against origin/develop\nelse\n  python3 scripts/check_migration_safety.py --allow-medium-risk\nfi\n"