feat(transcription): DTOs accept mentionedPersons sidecar with @Valid cascade

CreateTranscriptionBlockDTO and UpdateTranscriptionBlockDTO gain a
List<PersonMention> mentionedPersons field. @Valid is on the field itself,
not just on the controller method, so JSR-303 recurses into the list
elements when the controller boundary calls @Valid on the @RequestBody. The
collection defaults to an empty ArrayList via @Builder.Default; existing
constructor call sites in TranscriptionServiceTest are extended with
List.of() to match the new @AllArgsConstructor signature.

The controller-side @Valid wiring lands in the next commit alongside the
length-201 validation test.

Refs #362

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/src/main/java/org/raddatz/familienarchiv/dto/CreateTranscriptionBlockDTO.java

@@ -1,14 +1,21 @@
 package org.raddatz.familienarchiv.dto;
 
+import jakarta.validation.Valid;
 import jakarta.validation.constraints.Min;
 import jakarta.validation.constraints.Positive;
 import lombok.AllArgsConstructor;
+import lombok.Builder;
 import lombok.Data;
 import lombok.NoArgsConstructor;
+import org.raddatz.familienarchiv.model.PersonMention;
+
+import java.util.ArrayList;
+import java.util.List;
 
 @Data
 @NoArgsConstructor
 @AllArgsConstructor
+@Builder
 public class CreateTranscriptionBlockDTO {
     @Min(0)
     private int pageNumber;
@@ -22,4 +29,8 @@ public class CreateTranscriptionBlockDTO {
     private double height;
     private String text;
     private String label;
+
+    @Valid
+    @Builder.Default
+    private List<PersonMention> mentionedPersons = new ArrayList<>();
 }

backend/src/main/java/org/raddatz/familienarchiv/dto/UpdateTranscriptionBlockDTO.java

@@ -1,13 +1,24 @@
 package org.raddatz.familienarchiv.dto;
 
+import jakarta.validation.Valid;
 import lombok.AllArgsConstructor;
+import lombok.Builder;
 import lombok.Data;
 import lombok.NoArgsConstructor;
+import org.raddatz.familienarchiv.model.PersonMention;
+
+import java.util.ArrayList;
+import java.util.List;
 
 @Data
 @NoArgsConstructor
 @AllArgsConstructor
+@Builder
 public class UpdateTranscriptionBlockDTO {
     private String text;
     private String label;
+
+    @Valid
+    @Builder.Default
+    private List<PersonMention> mentionedPersons = new ArrayList<>();
 }