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_test.go

@@ -0,0 +1,28 @@
+//go:build unit
+
+package repository
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"github.com/redis/go-redis/v9"
+	"github.com/stretchr/testify/require"
+)
+
+func TestGeminiTokenCache_DeleteAccessToken_RedisError(t *testing.T) {
+	rdb := redis.NewClient(&redis.Options{
+		Addr:         "127.0.0.1:1",
+		DialTimeout:  50 * time.Millisecond,
+		ReadTimeout:  50 * time.Millisecond,
+		WriteTimeout: 50 * time.Millisecond,
+	})
+	t.Cleanup(func() {
+		_ = rdb.Close()
+	})
+
+	cache := NewGeminiTokenCache(rdb)
+	err := cache.DeleteAccessToken(context.Background(), "broken")
+	require.Error(t, err)
+}