fix(认证): 修复 OAuth token 缓存失效与 401 处理

新增 token 缓存失效接口并在刷新后清理
401 限流支持自定义规则与可配置冷却时间
补齐缓存失效与 401 处理测试
测试: make test

backend/internal/repository/gemini_token_cache_integration_test.go

@@ -0,0 +1,47 @@
+//go:build integration
+
+package repository
+
+import (
+	"errors"
+	"testing"
+	"time"
+
+	"github.com/Wei-Shaw/sub2api/internal/service"
+	"github.com/redis/go-redis/v9"
+	"github.com/stretchr/testify/require"
+	"github.com/stretchr/testify/suite"
+)
+
+type GeminiTokenCacheSuite struct {
+	IntegrationRedisSuite
+	cache service.GeminiTokenCache
+}
+
+func (s *GeminiTokenCacheSuite) SetupTest() {
+	s.IntegrationRedisSuite.SetupTest()
+	s.cache = NewGeminiTokenCache(s.rdb)
+}
+
+func (s *GeminiTokenCacheSuite) TestDeleteAccessToken() {
+	cacheKey := "project-123"
+	token := "token-value"
+	require.NoError(s.T(), s.cache.SetAccessToken(s.ctx, cacheKey, token, time.Minute))
+
+	got, err := s.cache.GetAccessToken(s.ctx, cacheKey)
+	require.NoError(s.T(), err)
+	require.Equal(s.T(), token, got)
+
+	require.NoError(s.T(), s.cache.DeleteAccessToken(s.ctx, cacheKey))
+
+	_, err = s.cache.GetAccessToken(s.ctx, cacheKey)
+	require.True(s.T(), errors.Is(err, redis.Nil), "expected redis.Nil after delete")
+}
+
+func (s *GeminiTokenCacheSuite) TestDeleteAccessToken_MissingKey() {
+	require.NoError(s.T(), s.cache.DeleteAccessToken(s.ctx, "missing-key"))
+}
+
+func TestGeminiTokenCacheSuite(t *testing.T) {
+	suite.Run(t, new(GeminiTokenCacheSuite))
+}