✨ Add removeFromMine functionality to Meetups API for removing meetups from a user's "My Meetups" list

- 🔒 Introduce `removeFromMine` policy for authenticated users to remove meetups.
- ✏️ Add `removeFromMine` method in `MeetupController` with idempotent handling.
- ✨ Add `removeMember` utility in `Meetup` model for managing pivot relationships.
- 🧪 Add feature tests for `removeFromMine`, covering idempotency, permissions, and unknown slugs.
- 🌐 Register `removeFromMine` route in API and link it to `MeetupController`.

app/Http/Controllers/Api/MeetupController.php

@@ -144,6 +144,24 @@ class MeetupController extends Controller
                 : \Symfony\Component\HttpFoundation\Response::HTTP_OK);
     }
 
+    /**
+     * Meetup aus „Meine Meetups" entfernen
+     *
+     * Entfernt ein Meetup aus der „Meine Meetups"-Liste des authentifizierten Nutzers
+     * (löst die meetup_user-Pivot-Mitgliedschaft). Die Stammdaten des Meetups bleiben
+     * erhalten — Gegenstück zu addToMine(). Idempotent: war das Meetup nicht (mehr)
+     * zugeordnet, bleibt die Antwort 200 OK.
+     */
+    #[Response(status: 401, description: 'Nicht authentifiziert.')]
+    public function removeFromMine(Request $request, Meetup $meetup): MeetupResource
+    {
+        Gate::authorize('removeFromMine', $meetup);
+
+        $meetup->removeMember($request->user());
+
+        return MeetupResource::make($meetup);
+    }
+
     /**
      * Eigenes Meetup anzeigen
      *