feat(config): add jackson-databind-nullable for JsonNullable PATCH DTO support

Registers JsonNullableModule globally so JsonNullable<String> in
JourneyItemUpdateDTO can distinguish absent (unchanged) from explicit
null (clear field) on PATCH operations.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/main/java/org/raddatz/familienarchiv/config/JacksonConfig.java

@@ -0,0 +1,16 @@
+package org.raddatz.familienarchiv.config;
+
+import com.fasterxml.jackson.databind.ObjectMapper;
+import org.openapitools.jackson.nullable.JsonNullableModule;
+import org.springframework.boot.autoconfigure.jackson.Jackson2ObjectMapperBuilderCustomizer;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+
+@Configuration
+public class JacksonConfig {
+
+    @Bean
+    public Jackson2ObjectMapperBuilderCustomizer jsonNullableModule() {
+        return builder -> builder.modulesToInstall(new JsonNullableModule());
+    }
+}