feat(server): workflow log-writer registry, retention setting, sweeper

server/internal/models/settings.go

@@ -36,4 +36,6 @@ type Settings struct {
 	Alerts  AlertSettings   `bson:"alerts" json:"alerts"`
 	Email   EmailSettings   `bson:"email" json:"email"`
 	Secrets SecretsSettings `bson:"secrets" json:"secrets"`
+	// WorkflowLogRetentionDays: nil = default 30, 0 = keep forever, N = N days.
+	WorkflowLogRetentionDays *int `bson:"workflow_log_retention_days,omitempty" json:"workflow_log_retention_days,omitempty"`
 }