✨ 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/Models/User.php

@@ -105,7 +105,7 @@ class User extends Authenticatable implements CipherSweetEncrypted
 
     public function meetups()
     {
-        return $this->belongsToMany(Meetup::class);
+        return $this->belongsToMany(Meetup::class)->withPivot('is_leader');
     }
 
     public function reputations()