feat(service): 定义 Gemini 服务端口接口

- 定义 OAuth 服务接口
- 定义 Token 缓存服务接口
- 定义 Code Assist 服务接口

backend/internal/service/gemini_oauth.go

@@ -0,0 +1,13 @@
+package ports
+
+import (
+	"context"
+
+	"github.com/Wei-Shaw/sub2api/internal/pkg/geminicli"
+)
+
+// GeminiOAuthClient performs Google OAuth token exchange/refresh for Gemini integration.
+type GeminiOAuthClient interface {
+	ExchangeCode(ctx context.Context, code, codeVerifier, redirectURI, proxyURL string) (*geminicli.TokenResponse, error)
+	RefreshToken(ctx context.Context, refreshToken, proxyURL string) (*geminicli.TokenResponse, error)
+}