build: 升级 Go 1.27.0，同步 CI/Dockerfile 并适配 jsonv2 与 golangci-lint v2.13

- go.mod 1.26.6 → 1.27.0；backend-ci/release/security-scan 的 go version 断言、
  三个 Dockerfile 的 golang 镜像、README 徽章与 DEV_GUIDE 同步
- golangci-lint-action v2.9 → v2.13（v2.9 由 go1.26 构建，拒绝 go.mod 1.27 目标）；
  新规则按最小方式处理：排除 G703/G704 污点分析（网关按配置转发/写文件，
  与既有 G304 排除策略一致）、reflect.Ptr → reflect.Pointer、
  ResetQuota 恒返回错误的 SA4023 与 OIDC EC JWK 的 SA1019 加 nolint
- ent 生成代码按 Go 1.27 默认 jsonv2 引擎重新生成：json.RawMessage 字段
  生成为同类型别名 jsontext.Value（group.model_pricing / usage_cleanup_task.filters）
- x/net v0.56 在 go1.27 下包装标准库 HTTP/2：ConfigureTransports 经
  RegisterProtocol("http/2") 打开 Protocols.HTTP2 而不再写 TLSNextProto，
  ReadIdleTimeout/PingTimeout 建连时映射为 HTTP2Config.SendPingTimeout/PingTimeout；
  keepalive 测试改断言 Protocols.HTTP2()，并补真实 HTTP/2 协商用例

.github/workflows/backend-ci.yml

@@ -33,7 +33,7 @@ jobs:
           cache-dependency-path: backend/go.sum
       - name: Verify Go version
         run: |
-          go version | grep -q 'go1.26.6'
+          go version | grep -q 'go1.27.0'
       - name: Unit tests
         working-directory: backend
         run: make test-unit
@@ -73,10 +73,10 @@ jobs:
           cache-dependency-path: backend/go.sum
       - name: Verify Go version
         run: |
-          go version | grep -q 'go1.26.6'
+          go version | grep -q 'go1.27.0'
       - name: golangci-lint
         uses: golangci/golangci-lint-action@v9
         with:
-          version: v2.9
+          version: v2.13
           args: --timeout=30m
           working-directory: backend

.github/workflows/release.yml

@@ -115,7 +115,7 @@ jobs:
 
       - name: Verify Go version
         run: |
-          go version | grep -q 'go1.26.6'
+          go version | grep -q 'go1.27.0'
 
       # Docker setup for GoReleaser
       - name: Set up QEMU

.github/workflows/security-scan.yml

@@ -23,7 +23,7 @@ jobs:
           cache-dependency-path: backend/go.sum
       - name: Verify Go version
         run: |
-          go version | grep -q 'go1.26.6'
+          go version | grep -q 'go1.27.0'
       - name: Run govulncheck
         working-directory: backend
         run: |