test(#148): add PersonController, DocumentSpecifications, and PersonRepository tests

- PersonControllerTest: expand from 2 to 26 tests — covers all endpoints
  (GET persons/id/correspondents/documents, POST create/merge, PUT update)
  and all validation branches (missing/blank firstName, lastName,
  targetPersonId → 400). Reveals and fixes a real bug: ResponseStatusException
  thrown by controllers was caught by the catch-all ExceptionHandler(Exception)
  in GlobalExceptionHandler, returning 500 instead of the intended status.
  Fix: add explicit ExceptionHandler(ResponseStatusException) handler.

- DocumentSpecificationsTest: 18 @DataJpaTest tests covering every branch in
  DocumentSpecifications (hasText null/blank/match/case, hasSender null/match,
  hasReceiver null/match, isBetween both-null/both-set/start-only/end-only,
  hasTags null/empty/match/AND-logic/case/whitespace-skip). This is the
  primary driver of the 0% repository branch coverage reported in #148.

- PersonRepositoryTest: 10 new tests for previously untested native queries —
  findCorrespondents (order by doc count), findCorrespondentsWithFilter
  (case-insensitive), reassignSender, insertMissingReceiverReference
  (no-duplicate guard), deleteReceiverReferences.

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

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

@@ -8,6 +8,7 @@ import org.springframework.http.ResponseEntity;
 import org.springframework.web.bind.MethodArgumentNotValidException;
 import org.springframework.web.bind.annotation.ExceptionHandler;
 import org.springframework.web.bind.annotation.RestControllerAdvice;
+import org.springframework.web.server.ResponseStatusException;
 
 import lombok.extern.slf4j.Slf4j;
 
@@ -30,6 +31,12 @@ public class GlobalExceptionHandler {
         return ResponseEntity.badRequest().body(new ErrorResponse(ErrorCode.VALIDATION_ERROR, message));
     }
 
+    @ExceptionHandler(ResponseStatusException.class)
+    public ResponseEntity<ErrorResponse> handleResponseStatus(ResponseStatusException ex) {
+        return ResponseEntity.status(ex.getStatusCode())
+                .body(new ErrorResponse(ErrorCode.VALIDATION_ERROR, ex.getReason()));
+    }
+
     @ExceptionHandler(Exception.class)
     public ResponseEntity<ErrorResponse> handleGeneric(Exception ex) {
         log.error("Unhandled exception", ex);