feat(backend): 添加 Gemini V1beta Handler 和路由

- 新增 gemini_v1beta_handler.go: 代理原生 Google API 格式
- 更新 gemini_oauth_handler.go: 移除 redirectUri，新增 oauthType
- 更新 account_handler.go: 账户 Handler 增强
- 更新 router.go: 注册 v1beta 路由
- 更新 config.go: Gemini OAuth 通过环境变量配置
- 更新 wire_gen.go: 依赖注入

backend/internal/handler/admin/account_handler.go

@@ -6,6 +6,7 @@ import (
 
 	"github.com/Wei-Shaw/sub2api/internal/model"
 	"github.com/Wei-Shaw/sub2api/internal/pkg/claude"
+	"github.com/Wei-Shaw/sub2api/internal/pkg/geminicli"
 	"github.com/Wei-Shaw/sub2api/internal/pkg/openai"
 	"github.com/Wei-Shaw/sub2api/internal/pkg/response"
 	"github.com/Wei-Shaw/sub2api/internal/pkg/timezone"
@@ -879,6 +880,44 @@ func (h *AccountHandler) GetAvailableModels(c *gin.Context) {
 		return
 	}
 
+	// Handle Gemini accounts
+	if account.IsGemini() {
+		// For OAuth accounts: return default Gemini models
+		if account.IsOAuth() {
+			response.Success(c, geminicli.DefaultModels)
+			return
+		}
+
+		// For API Key accounts: return models based on model_mapping
+		mapping := account.GetModelMapping()
+		if len(mapping) == 0 {
+			response.Success(c, geminicli.DefaultModels)
+			return
+		}
+
+		var models []geminicli.Model
+		for requestedModel := range mapping {
+			var found bool
+			for _, dm := range geminicli.DefaultModels {
+				if dm.ID == requestedModel {
+					models = append(models, dm)
+					found = true
+					break
+				}
+			}
+			if !found {
+				models = append(models, geminicli.Model{
+					ID:          requestedModel,
+					Type:        "model",
+					DisplayName: requestedModel,
+					CreatedAt:   "",
+				})
+			}
+		}
+		response.Success(c, models)
+		return
+	}
+
 	// Handle Claude/Anthropic accounts
 	// For OAuth and Setup-Token accounts: return default models
 	if account.IsOAuth() {