feat(dashboard): expose youParticipated in ActivityFeedItemDTO

Add youParticipated field to the DTO record and wire row.isYouParticipated()
through DashboardService.getActivity().

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

backend/src/main/java/org/raddatz/familienarchiv/dashboard/ActivityFeedItemDTO.java

@@ -15,6 +15,7 @@ public record ActivityFeedItemDTO(
         @Schema(requiredMode = Schema.RequiredMode.REQUIRED) String documentTitle,
         @Schema(requiredMode = Schema.RequiredMode.REQUIRED) OffsetDateTime happenedAt,
         @Schema(requiredMode = Schema.RequiredMode.REQUIRED) boolean youMentioned,
+        @Schema(requiredMode = Schema.RequiredMode.REQUIRED) boolean youParticipated,
         @Schema(requiredMode = Schema.RequiredMode.REQUIRED) int count,
         @Nullable OffsetDateTime happenedAtUntil
 ) {}

backend/src/main/java/org/raddatz/familienarchiv/dashboard/DashboardService.java

@@ -140,6 +140,7 @@ public class DashboardService {
                     docTitle,
                     row.getHappenedAt().atOffset(ZoneOffset.UTC),
                     row.isYouMentioned(),
+                    row.isYouParticipated(),
                     row.getCount(),
                     happenedAtUntil
             );