Make Transaction serializable

entity/src/main/kotlin/me/madhead/tyzenhaus/entity/serializers/InstantSerializer.kt

@@ -0,0 +1,18 @@
+package me.madhead.tyzenhaus.entity.serializers
+
+import java.time.Instant
+import kotlinx.serialization.KSerializer
+import kotlinx.serialization.descriptors.PrimitiveKind
+import kotlinx.serialization.descriptors.PrimitiveSerialDescriptor
+import kotlinx.serialization.descriptors.SerialDescriptor
+import kotlinx.serialization.encoding.Decoder
+import kotlinx.serialization.encoding.Encoder
+
+/**
+ * [KSerializer] for [Instant]s.
+ */
+class InstantSerializer : KSerializer<Instant> {
+    override val descriptor: SerialDescriptor = PrimitiveSerialDescriptor("Instant", PrimitiveKind.INT)
+    override fun serialize(encoder: Encoder, value: Instant) = encoder.encodeLong(value.toEpochMilli())
+    override fun deserialize(decoder: Decoder): Instant = Instant.ofEpochMilli(decoder.decodeLong())
+}

entity/src/main/kotlin/me/madhead/tyzenhaus/entity/transaction/Transaction.kt

@@ -2,17 +2,23 @@ package me.madhead.tyzenhaus.entity.transaction
 
 import java.math.BigDecimal
 import java.time.Instant
+import kotlinx.serialization.Serializable
+import me.madhead.tyzenhaus.entity.serializers.BigDecimalSerializer
+import me.madhead.tyzenhaus.entity.serializers.InstantSerializer
 
 /**
  * Shared expense.
  */
+@Serializable
 data class Transaction(
     val id: Long?,
     val groupId: Long,
     val payer: Long,
     val recipients: Set<Long>,
+    @Serializable(BigDecimalSerializer::class)
     val amount: BigDecimal,
     val currency: String,
     val title: String? = null,
+    @Serializable(InstantSerializer::class)
     val timestamp: Instant = Instant.now(),
 )