Nest extraData by section for clearer embedding content

Homebox's location fields were flattened into single-purpose keys
(locationName/locationDescription), which gave EmbeddingService no
way to group related fields when rendering vector-store content.
extraData is now Map<String, Map<String, Object>>, letting
HomeboxItemClient express "location" as its own section; buildExtraData
renders each section as a header with indented fields instead of a flat
"Extradata: key: value" line.

backend/src/main/java/com/vaessl/app/shared/ServiceItem.java

@@ -4,7 +4,7 @@ import java.util.Map;
 import jakarta.validation.constraints.NotNull;
 
 public record ServiceItem(String id, @NotNull String title, String description,
-        Map<String, Object> extraData) {
+        Map<String, Map<String, Object>> extraData) {
 
     public String getExtra(String key) {
         if (extraData == null) {