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/geminicli/constants.go

@@ -9,9 +9,23 @@ const (
 	AuthorizeURL = "https://accounts.google.com/o/oauth2/v2/auth"
 	TokenURL     = "https://oauth2.googleapis.com/token"
 
-	// DefaultScopes is the minimal scope set for GeminiCli/CodeAssist usage.
-	// Keep this conservative and expand only when we have a clear requirement.
-	DefaultScopes = "https://www.googleapis.com/auth/cloud-platform"
+	// AIStudioOAuthRedirectURI is the default redirect URI used for AI Studio OAuth.
+	// This matches the "copy/paste callback URL" flow used by OpenAI OAuth in this project.
+	// Note: You still need to register this redirect URI in your Google OAuth client
+	// unless you use an OAuth client type that permits localhost redirect URIs.
+	AIStudioOAuthRedirectURI = "http://localhost:1455/auth/callback"
+
+	// DefaultScopes for Code Assist (includes cloud-platform for API access plus userinfo scopes)
+	// Required by Google's Code Assist API.
+	DefaultCodeAssistScopes = "https://www.googleapis.com/auth/cloud-platform https://www.googleapis.com/auth/userinfo.email https://www.googleapis.com/auth/userinfo.profile"
+
+	// DefaultScopes for AI Studio (uses generativelanguage API with OAuth)
+	// Reference: https://ai.google.dev/gemini-api/docs/oauth
+	// For regular Google accounts, supports API calls to generativelanguage.googleapis.com
+	DefaultAIStudioScopes = "https://www.googleapis.com/auth/cloud-platform https://www.googleapis.com/auth/generative-language"
+
+	// GeminiCLIRedirectURI is the redirect URI used by Gemini CLI for Code Assist OAuth.
+	GeminiCLIRedirectURI = "https://codeassist.google.com/authcode"
 
 	SessionTTL = 30 * time.Minute