feat(service): 实现 Gemini OAuth 和 Token 管理服务

- 实现 OAuth 授权流程服务
- 添加 Token 提供者和自动刷新机制
- 实现 Gemini Messages API 兼容层
- 更新服务容器注册

backend/internal/service/gemini_token_refresher.go

@@ -0,0 +1,53 @@
+package service
+
+import (
+	"context"
+	"strconv"
+	"time"
+
+	"github.com/Wei-Shaw/sub2api/internal/model"
+)
+
+type GeminiTokenRefresher struct {
+	geminiOAuthService *GeminiOAuthService
+}
+
+func NewGeminiTokenRefresher(geminiOAuthService *GeminiOAuthService) *GeminiTokenRefresher {
+	return &GeminiTokenRefresher{geminiOAuthService: geminiOAuthService}
+}
+
+func (r *GeminiTokenRefresher) CanRefresh(account *model.Account) bool {
+	return account.Platform == model.PlatformGemini && account.Type == model.AccountTypeOAuth
+}
+
+func (r *GeminiTokenRefresher) NeedsRefresh(account *model.Account, refreshWindow time.Duration) bool {
+	if !r.CanRefresh(account) {
+		return false
+	}
+	expiresAtStr := account.GetCredential("expires_at")
+	if expiresAtStr == "" {
+		return false
+	}
+	expiresAt, err := strconv.ParseInt(expiresAtStr, 10, 64)
+	if err != nil {
+		return false
+	}
+	expiryTime := time.Unix(expiresAt, 0)
+	return time.Until(expiryTime) < refreshWindow
+}
+
+func (r *GeminiTokenRefresher) Refresh(ctx context.Context, account *model.Account) (map[string]any, error) {
+	tokenInfo, err := r.geminiOAuthService.RefreshAccountToken(ctx, account)
+	if err != nil {
+		return nil, err
+	}
+
+	newCredentials := r.geminiOAuthService.BuildAccountCredentials(tokenInfo)
+	for k, v := range account.Credentials {
+		if _, exists := newCredentials[k]; !exists {
+			newCredentials[k] = v
+		}
+	}
+
+	return newCredentials, nil
+}