fix(ci): 将 gosec 集成到 golangci-lint 解决安全扫描超时

standalone gosec 扫描 24 万行 Go 代码在 CI 中持续超时，
将其作为 golangci-lint 的内置 linter 运行，复用 AST 解析和缓存大幅提速。

- 在 .golangci.yml 中启用 gosec 并迁移原有排除规则
- golangci-lint timeout 从 5m 提升到 30m
- 从 security-scan.yml 移除 standalone gosec 步骤
- 删除不再需要的 .gosec.json 配置文件

.github/workflows/security-scan.yml

@@ -29,12 +29,6 @@ jobs:
         run: |
           go install golang.org/x/vuln/cmd/govulncheck@latest
           govulncheck ./...
-      - name: Run gosec
-        working-directory: backend
-        run: |
-          go install github.com/securego/gosec/v2/cmd/gosec@latest
-          # exclude ent/ — auto-generated ORM code, not subject to manual security review
-          gosec -conf .gosec.json -severity high -confidence high -exclude-generated -exclude-dir=ent ./...
 
   frontend-security:
     runs-on: ubuntu-latest