feat(backend): 添加 Gemini/Google API 基础包

- 新增 pkg/gemini: 模型定义与回退列表
- 新增 pkg/googleapi: Google API 错误状态处理
- 新增 pkg/geminicli/models.go: CLI 模型结构
- 更新 constants.go: AI Studio 相关常量
- 更新 oauth.go: 支持 AI Studio OAuth 流程，凭据通过环境变量配置

backend/internal/pkg/googleapi/status.go

@@ -0,0 +1,25 @@
+package googleapi
+
+import "net/http"
+
+// HTTPStatusToGoogleStatus maps HTTP status codes to Google-style error status strings.
+func HTTPStatusToGoogleStatus(status int) string {
+	switch status {
+	case http.StatusBadRequest:
+		return "INVALID_ARGUMENT"
+	case http.StatusUnauthorized:
+		return "UNAUTHENTICATED"
+	case http.StatusForbidden:
+		return "PERMISSION_DENIED"
+	case http.StatusNotFound:
+		return "NOT_FOUND"
+	case http.StatusTooManyRequests:
+		return "RESOURCE_EXHAUSTED"
+	default:
+		if status >= 500 {
+			return "INTERNAL"
+		}
+		return "UNKNOWN"
+	}
+}
+