Audit Event Entity
data class AuditEventEntity(val id: String, val ts: Long, val name: String, val fieldsJson: String, val prevHash: String, val hash: String, val signature: String? = null, val deviceAttestationBlob: ByteArray? = null, val chainGeneration: Int = 1, val userId: String? = null)
Room entity for persistent audit events.
Each event is part of a hash chain, where hash includes the prevHash to create a tamper-evident log structure.