chore: Go 工具链升级 1.26.4 → 1.26.5——修复 stdlib 漏洞并补齐 CI 版本引用

- backend/go.mod 工具链 1.26.5：修复 stdlib crypto/tls 漏洞（GO-2026-5856）
- 同步全部构建/校验点的硬编码版本：根 Dockerfile、backend/Dockerfile、
  deploy/Dockerfile 基础镜像；backend-ci / release / security-scan 三个
  workflow 的 go version 校验

.github/workflows/backend-ci.yml

@@ -20,7 +20,7 @@ jobs:
           cache-dependency-path: backend/go.sum
       - name: Verify Go version
         run: |
-          go version | grep -q 'go1.26.4'
+          go version | grep -q 'go1.26.5'
       - name: Unit tests
         working-directory: backend
         run: make test-unit
@@ -60,7 +60,7 @@ jobs:
           cache-dependency-path: backend/go.sum
       - name: Verify Go version
         run: |
-          go version | grep -q 'go1.26.4'
+          go version | grep -q 'go1.26.5'
       - name: golangci-lint
         uses: golangci/golangci-lint-action@v9
         with:

.github/workflows/release.yml

@@ -115,7 +115,7 @@ jobs:
 
       - name: Verify Go version
         run: |
-          go version | grep -q 'go1.26.4'
+          go version | grep -q 'go1.26.5'
 
       # 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 'go1.26.4'
+          go version | grep -q 'go1.26.5'
       - name: Run govulncheck
         working-directory: backend
         run: |