perf(认证): 引入 API Key 认证缓存与轻量删除查询

增加 L1/L2 缓存、负缓存与单飞回源
使用 key+owner 轻量查询替代全量加载并清理旧接口
补充缓存失效与余额更新测试，修复随机抖动 lint

测试: make test

backend/internal/service/api_key_auth_cache_invalidate.go

@@ -0,0 +1,48 @@
+package service
+
+import "context"
+
+// InvalidateAuthCacheByKey 清除指定 API Key 的认证缓存
+func (s *APIKeyService) InvalidateAuthCacheByKey(ctx context.Context, key string) {
+	if key == "" {
+		return
+	}
+	cacheKey := s.authCacheKey(key)
+	s.deleteAuthCache(ctx, cacheKey)
+}
+
+// InvalidateAuthCacheByUserID 清除用户相关的 API Key 认证缓存
+func (s *APIKeyService) InvalidateAuthCacheByUserID(ctx context.Context, userID int64) {
+	if userID <= 0 {
+		return
+	}
+	keys, err := s.apiKeyRepo.ListKeysByUserID(ctx, userID)
+	if err != nil {
+		return
+	}
+	s.deleteAuthCacheByKeys(ctx, keys)
+}
+
+// InvalidateAuthCacheByGroupID 清除分组相关的 API Key 认证缓存
+func (s *APIKeyService) InvalidateAuthCacheByGroupID(ctx context.Context, groupID int64) {
+	if groupID <= 0 {
+		return
+	}
+	keys, err := s.apiKeyRepo.ListKeysByGroupID(ctx, groupID)
+	if err != nil {
+		return
+	}
+	s.deleteAuthCacheByKeys(ctx, keys)
+}
+
+func (s *APIKeyService) deleteAuthCacheByKeys(ctx context.Context, keys []string) {
+	if len(keys) == 0 {
+		return
+	}
+	for _, key := range keys {
+		if key == "" {
+			continue
+		}
+		s.deleteAuthCache(ctx, s.authCacheKey(key))
+	}
+}