fix(backend): add LocalDate serialization module

backend/src/main/kotlin/org/loculus/backend/utils/LocalDateJacksonModule.kt

@@ -0,0 +1,29 @@
+package org.loculus.backend.utils
+
+import com.fasterxml.jackson.core.JsonGenerator
+import com.fasterxml.jackson.core.JsonParser
+import com.fasterxml.jackson.databind.DeserializationContext
+import com.fasterxml.jackson.databind.JsonDeserializer
+import com.fasterxml.jackson.databind.JsonSerializer
+import com.fasterxml.jackson.databind.SerializerProvider
+import com.fasterxml.jackson.databind.module.SimpleModule
+import kotlinx.datetime.LocalDate
+import org.springframework.context.annotation.Configuration
+
+@Configuration
+class LocalDateJacksonModule : SimpleModule() {
+    init {
+        addSerializer(LocalDate::class.java, LocalDateSerializer())
+        addDeserializer(LocalDate::class.java, LocalDateDeserializer())
+    }
+}
+
+class LocalDateSerializer : JsonSerializer<LocalDate>() {
+    override fun serialize(value: LocalDate, gen: JsonGenerator, serializers: SerializerProvider) {
+        gen.writeString(value.toString())
+    }
+}
+
+class LocalDateDeserializer : JsonDeserializer<LocalDate>() {
+    override fun deserialize(p: JsonParser, ctxt: DeserializationContext): LocalDate = LocalDate.parse(p.text)
+}