fix(lint): 修复所有 Go 命名规范问题

- 全局替换 ApiKey → APIKey（类型、字段、方法、变量）
- 修复所有 initialism 命名（API, SMTP, HTML, URL 等）
- 添加所有缺失的包注释
- 修复导出符号的注释格式

主要修改：
- ApiKey → APIKey（所有出现的地方）
- ApiKeyID → APIKeyID
- ApiKeyIDs → APIKeyIDs
- TestSmtpConnection → TestSMTPConnection
- HtmlURL → HTMLURL
- 添加 20+ 个包注释
- 修复 10+ 个导出符号注释格式

验证结果：
- ✓ golangci-lint: 0 issues
- ✓ 单元测试: 通过
- ✓ 集成测试: 通过

backend/internal/service/dashboard_service.go

@@ -43,8 +43,8 @@ func (s *DashboardService) GetModelStatsWithFilters(ctx context.Context, startTi
 	return stats, nil
 }
 
-func (s *DashboardService) GetApiKeyUsageTrend(ctx context.Context, startTime, endTime time.Time, granularity string, limit int) ([]usagestats.ApiKeyUsageTrendPoint, error) {
-	trend, err := s.usageRepo.GetApiKeyUsageTrend(ctx, startTime, endTime, granularity, limit)
+func (s *DashboardService) GetAPIKeyUsageTrend(ctx context.Context, startTime, endTime time.Time, granularity string, limit int) ([]usagestats.APIKeyUsageTrendPoint, error) {
+	trend, err := s.usageRepo.GetAPIKeyUsageTrend(ctx, startTime, endTime, granularity, limit)
 	if err != nil {
 		return nil, fmt.Errorf("get api key usage trend: %w", err)
 	}
@@ -67,8 +67,8 @@ func (s *DashboardService) GetBatchUserUsageStats(ctx context.Context, userIDs [
 	return stats, nil
 }
 
-func (s *DashboardService) GetBatchApiKeyUsageStats(ctx context.Context, apiKeyIDs []int64) (map[int64]*usagestats.BatchApiKeyUsageStats, error) {
-	stats, err := s.usageRepo.GetBatchApiKeyUsageStats(ctx, apiKeyIDs)
+func (s *DashboardService) GetBatchAPIKeyUsageStats(ctx context.Context, apiKeyIDs []int64) (map[int64]*usagestats.BatchAPIKeyUsageStats, error) {
+	stats, err := s.usageRepo.GetBatchAPIKeyUsageStats(ctx, apiKeyIDs)
 	if err != nil {
 		return nil, fmt.Errorf("get batch api key usage stats: %w", err)
 	}