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

+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)
+}

backend/internal/service/gemini_token_cache.go

+package ports
+
+import (
+	"context"
+	"time"
+)
+
+// GeminiTokenCache stores short-lived access tokens and coordinates refresh to avoid stampedes.
+type GeminiTokenCache interface {
+	// cacheKey should be stable for the token scope; for GeminiCli OAuth we primarily use project_id.
+	GetAccessToken(ctx context.Context, cacheKey string) (string, error)
+	SetAccessToken(ctx context.Context, cacheKey string, token string, ttl time.Duration) error
+
+	AcquireRefreshLock(ctx context.Context, cacheKey string, ttl time.Duration) (bool, error)
+	ReleaseRefreshLock(ctx context.Context, cacheKey string) error
+}

backend/internal/service/geminicli_codeassist.go

+package ports
+
+import (
+	"context"
+
+	"github.com/Wei-Shaw/sub2api/internal/pkg/geminicli"
+)
+
+// GeminiCliCodeAssistClient calls GeminiCli internal Code Assist endpoints.
+type GeminiCliCodeAssistClient interface {
+	LoadCodeAssist(ctx context.Context, accessToken, proxyURL string, req *geminicli.LoadCodeAssistRequest) (*geminicli.LoadCodeAssistResponse, error)
+	OnboardUser(ctx context.Context, accessToken, proxyURL string, req *geminicli.OnboardUserRequest) (*geminicli.OnboardUserResponse, error)
+}