feat(usage): add requested model usage metadata helpers

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/internal/service/usage_log.go

@@ -98,6 +98,9 @@ type UsageLog struct {
 	AccountID int64
 	RequestID string
 	Model     string
+	// RequestedModel is the client-requested model name recorded for stable user/admin display.
+	// Empty should be treated as Model for backward compatibility with historical rows.
+	RequestedModel string
 	// UpstreamModel is the actual model sent to the upstream provider after mapping.
 	// Nil means no mapping was applied (requested model was used as-is).
 	UpstreamModel *string