ci(lint): golangci-lint v2 入门禁并归零

- .golangci.yml:standard 集 - errcheck(存量 38 处裸忽略待专项),staticcheck 关纯风格/quickfix
- 清理 3 个无引用死函数;SanitizedJSON 不可达分支收敛为接口分支 + nil 指针防护(原死分支中的 nil 检查从未执行)
- CI backend job 增加 golangci-lint-action@v6

.gitea/workflows/ci.yml

@@ -27,6 +27,10 @@ jobs:
       - name: go vet
         working-directory: backend
         run: go vet ./...
+      - name: golangci-lint
+        uses: golangci/golangci-lint-action@v6
+        with:
+          working-directory: backend
       - name: go test
         working-directory: backend
         run: go test ./...