feat(persons): add @Size constraints to PersonUpdateDTO + @Valid to controller

firstName/lastName max 100, alias max 200, notes max 5000 chars.
PUT /api/persons/{id} returns 400 for oversized fields.

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

backend/src/main/java/org/raddatz/familienarchiv/controller/PersonController.java

@@ -13,9 +13,12 @@ import org.raddatz.familienarchiv.service.DocumentService;
 import org.raddatz.familienarchiv.service.PersonService;
 import org.springframework.http.HttpStatus;
 import org.springframework.http.ResponseEntity;
+import org.springframework.validation.annotation.Validated;
 import org.springframework.web.bind.annotation.*;
 import org.springframework.web.server.ResponseStatusException;
 
+import jakarta.validation.Valid;
+
 import lombok.RequiredArgsConstructor;
 
 @RestController
@@ -66,7 +69,7 @@ public class PersonController {
 
     @PutMapping("/{id}")
     @RequirePermission(Permission.WRITE_ALL)
-    public ResponseEntity<Person> updatePerson(@PathVariable UUID id, @RequestBody PersonUpdateDTO dto) {
+    public ResponseEntity<Person> updatePerson(@PathVariable UUID id, @Valid @RequestBody PersonUpdateDTO dto) {
         if (dto.getFirstName() == null || dto.getFirstName().isBlank()
                 || dto.getLastName() == null || dto.getLastName().isBlank()) {
             throw new ResponseStatusException(HttpStatus.BAD_REQUEST, "Vor- und Nachname sind Pflichtfelder");

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

@@ -1,12 +1,17 @@
 package org.raddatz.familienarchiv.dto;
 
+import jakarta.validation.constraints.Size;
 import lombok.Data;
 
 @Data
 public class PersonUpdateDTO {
+    @Size(max = 100)
     private String firstName;
+    @Size(max = 100)
     private String lastName;
+    @Size(max = 200)
     private String alias;
+    @Size(max = 5000)
     private String notes;
     private Integer birthYear;
     private Integer deathYear;