feat: festival/event model frontend + topbar activeren

- Events lijst: card grid met festival/serie chips
- Festival detail: programmaonderdelen grid
- CreateSubEventDialog voor sub-events binnen festival
- EventTabsNav: breadcrumb terug naar festival
- Sessie A: festival-bewuste EventResource + children endpoint
- Topbar: zoekbalk, theme switcher, shortcuts, notificaties
- Schema v1.7 + BACKLOG.md toegevoegd
- 121 tests groen

api/routes/api.php

@@ -55,6 +55,7 @@ Route::middleware('auth:sanctum')->group(function () {
     // Events (nested under organisations)
     Route::apiResource('organisations.events', EventController::class)
         ->only(['index', 'show', 'store', 'update']);
+    Route::get('organisations/{organisation}/events/{event}/children', [EventController::class, 'children']);
 
     // Organisation-scoped resources
     Route::prefix('organisations/{organisation}')->group(function () {