chore(documents): density endpoint produces=application/json (#385)

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

backend/src/main/java/org/raddatz/familienarchiv/document/DocumentController.java

@@ -390,7 +390,7 @@ public class DocumentController {
         return ResponseEntity.ok(documentService.searchDocuments(q, from, to, senderId, receiverId, tags, tagQ, status, sort, dir, operator, pageable));
     }
 
-    @GetMapping("/density")
+    @GetMapping(value = "/density", produces = MediaType.APPLICATION_JSON_VALUE)
     public ResponseEntity<DocumentDensityResult> density(
             @RequestParam(required = false) String q,
             @RequestParam(required = false) UUID senderId,