✨ Add RSVP functionality for Meetup Events

- 🏷️ Introduce `RsvpStatus` enum for managing attendance states (`attending`, `maybe`, `none`).
- ✏️ Add `MeetupEventController` methods for RSVP actions (`rsvpStatus`, `rsvp`) and payload handling.
- ✨ Implement RSVP helpers in `MeetupEvent` model for user-specific attendance management.
- 🌐 Register RSVP routes for showing and updating attendance in the API.
- 🧪 Add feature tests for RSVP actions, covering validation, idempotency, and correct list handling.

app/Http/Requests/Api/RsvpMeetupEventRequest.php

@@ -0,0 +1,26 @@
+<?php
+
+namespace App\Http\Requests\Api;
+
+use App\Enums\RsvpStatus;
+use Illuminate\Foundation\Http\FormRequest;
+use Illuminate\Validation\Rule;
+
+class RsvpMeetupEventRequest extends FormRequest
+{
+    public function authorize(): bool
+    {
+        // Jeder authentifizierte Nutzer darf für einen Termin zu-/absagen.
+        return $this->user() !== null;
+    }
+
+    /**
+     * @return array<string, array<int, mixed>>
+     */
+    public function rules(): array
+    {
+        return [
+            'status' => ['required', Rule::enum(RsvpStatus::class)],
+        ];
+    }
+}