fix: check POST data for validity

feat: also introduces a template for search

WebApp/DTOs/EventSearchDto.cs

@@ -0,0 +1,16 @@
+using System.ComponentModel.DataAnnotations;
+using WebApp.Entities;
+
+namespace WebApp.DTOs;
+
+// Output values in JSON file
+public record class EventSearchDto
+(
+    int? OrganisationId,
+    string? Title,
+    string? Description,
+    string? Location,
+    DateTime? EventDate,
+    ICollection<EventSkill>? EventSkills, // obecnie nie dotyczy
+    ICollection<EventRegistration>? EventRegistrations // obecnie nie dotyczy
+);