Huge refactor for backend and client

This is barely functionnal, but at least it can display some data without crashing
Most other functionnalities other than displaying events are broken, and so are the relative date operations, they need to be fixed asap

models/CalendarEvent.ts

@@ -0,0 +1,18 @@
+import type { Category } from './Category'
+import type { RPGDate } from './Date'
+
+export interface CalendarEvent {
+  id: number
+  title: string
+  startDate: RPGDate
+  endDate?: RPGDate
+  description?: string
+  category?: Category
+  hidden?: boolean
+  wiki?: string
+}
+
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+export function isCalendarEvent(object: any): object is CalendarEvent {
+  return 'startDate' in object
+}