feat(repository): 实现 Gemini OAuth 和 Token 缓存客户端

- 添加 Gemini OAuth 客户端实现
- 实现 Redis 基础的 Token 缓存
- 添加 gemini-cli Code Assist 客户端封装

backend/internal/repository/gemini_oauth_client.go

@@ -0,0 +1,84 @@
+package repository
+
+import (
+	"context"
+	"fmt"
+	"net/url"
+	"time"
+
+	"github.com/Wei-Shaw/sub2api/internal/config"
+	"github.com/Wei-Shaw/sub2api/internal/pkg/geminicli"
+	"github.com/Wei-Shaw/sub2api/internal/service/ports"
+
+	"github.com/imroc/req/v3"
+)
+
+type geminiOAuthClient struct {
+	tokenURL string
+	cfg      *config.Config
+}
+
+func NewGeminiOAuthClient(cfg *config.Config) ports.GeminiOAuthClient {
+	return &geminiOAuthClient{
+		tokenURL: geminicli.TokenURL,
+		cfg:      cfg,
+	}
+}
+
+func (c *geminiOAuthClient) ExchangeCode(ctx context.Context, code, codeVerifier, redirectURI, proxyURL string) (*geminicli.TokenResponse, error) {
+	client := createGeminiReqClient(proxyURL)
+
+	formData := url.Values{}
+	formData.Set("grant_type", "authorization_code")
+	formData.Set("client_id", c.cfg.Gemini.OAuth.ClientID)
+	formData.Set("client_secret", c.cfg.Gemini.OAuth.ClientSecret)
+	formData.Set("code", code)
+	formData.Set("code_verifier", codeVerifier)
+	formData.Set("redirect_uri", redirectURI)
+
+	var tokenResp geminicli.TokenResponse
+	resp, err := client.R().
+		SetContext(ctx).
+		SetFormDataFromValues(formData).
+		SetSuccessResult(&tokenResp).
+		Post(c.tokenURL)
+	if err != nil {
+		return nil, fmt.Errorf("request failed: %w", err)
+	}
+	if !resp.IsSuccessState() {
+		return nil, fmt.Errorf("token exchange failed: status %d, body: %s", resp.StatusCode, geminicli.SanitizeBodyForLogs(resp.String()))
+	}
+	return &tokenResp, nil
+}
+
+func (c *geminiOAuthClient) RefreshToken(ctx context.Context, refreshToken, proxyURL string) (*geminicli.TokenResponse, error) {
+	client := createGeminiReqClient(proxyURL)
+
+	formData := url.Values{}
+	formData.Set("grant_type", "refresh_token")
+	formData.Set("refresh_token", refreshToken)
+	formData.Set("client_id", c.cfg.Gemini.OAuth.ClientID)
+	formData.Set("client_secret", c.cfg.Gemini.OAuth.ClientSecret)
+
+	var tokenResp geminicli.TokenResponse
+	resp, err := client.R().
+		SetContext(ctx).
+		SetFormDataFromValues(formData).
+		SetSuccessResult(&tokenResp).
+		Post(c.tokenURL)
+	if err != nil {
+		return nil, fmt.Errorf("request failed: %w", err)
+	}
+	if !resp.IsSuccessState() {
+		return nil, fmt.Errorf("token refresh failed: status %d, body: %s", resp.StatusCode, geminicli.SanitizeBodyForLogs(resp.String()))
+	}
+	return &tokenResp, nil
+}
+
+func createGeminiReqClient(proxyURL string) *req.Client {
+	client := req.C().SetTimeout(60 * time.Second)
+	if proxyURL != "" {
+		client.SetProxyURL(proxyURL)
+	}
+	return client
+}