feat(config): 新增 Gemini 配置项和 geminicli 核心包

- 添加 Gemini OAuth 配置结构
- 实现 geminicli 包（OAuth、Token、CodeAssist 类型）
- 更新配置示例文件

backend/internal/pkg/geminicli/constants.go

@@ -0,0 +1,20 @@
+package geminicli
+
+import "time"
+
+const (
+	AIStudioBaseURL  = "https://generativelanguage.googleapis.com"
+	GeminiCliBaseURL = "https://cloudcode-pa.googleapis.com"
+
+	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"
+
+	SessionTTL = 30 * time.Minute
+
+	// GeminiCLIUserAgent mimics Gemini CLI to maximize compatibility with internal endpoints.
+	GeminiCLIUserAgent = "GeminiCLI/0.1.5 (Windows; AMD64)"
+)