✨ Implement leadership-based permissions for Meetup management

- 🔒 Restrict event creation, editing, and deletion to Meetup leaders (`is_leader`) and creators for consistency across APIs, frontend, and MCP.
- ➕ Add new APIs for leader delegation: assign/remove Meetup leaders via `meetup_user.is_leader`.
- 🛠️ Replace loose member checks with specific leadership checks in policies, controllers, and views.
- 🧪 Add exhaustive tests to ensure only eligible leaders execute critical actions (e.g., event creation/edit, Meetup updates).
- 🔄 Refactor pivot relationships and models (`leadByMe`, `isLeader`) for explicit leadership handling.
- ✨ Introduce artisan command `meetups:promote-existing-leaders` to transition legacy data.

app/Http/Controllers/Api/UserController.php

@@ -51,7 +51,9 @@ class UserController extends Controller
             'email' => $user->email,
             'nostr' => $user->nostr,
             'is_lecturer' => (bool) $user->is_lecturer,
-            'is_leader' => (bool) $user->is_leader,
+            // Leader-Rolle ist pro Meetup (meetup_user.is_leader); global = ist
+            // der Nutzer Leader IRGENDEINES Meetups. Treibt das Rollen-Badge.
+            'is_leader' => $user->meetups()->wherePivot('is_leader', true)->exists(),
             'avatar' => $user->profile_photo_url,
         ];
     }