refactor: 错误消息脱敏 + 后端 CI

- 新增 utils.InternalMsg:内部错误详情只进服务端日志,对外统一脱敏
  消息,不再向客户端泄露 GORM/SQL 实现细节;九个 controller 的
  err.Error() 出口全部切换
- 新增 .github/workflows/ci.yml:go build/vet/test(backend,go 1.25)

.github/workflows/ci.yml

@@ -0,0 +1,24 @@
+name: CI
+
+on:
+  push:
+    branches: [main, master]
+  pull_request:
+
+jobs:
+  backend:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-go@v5
+        with:
+          go-version: "1.25"
+      - name: Build
+        run: go build ./...
+        working-directory: backend
+      - name: Vet
+        run: go vet ./...
+        working-directory: backend
+      - name: Test
+        run: go test ./... -count=1 -coverprofile=coverage.out
+        working-directory: backend