✨ Add leader management functionality for Meetups

- ➕ Introduced `leaders`, `promoteLeader`, and `demoteLeader` methods in `Meetup` model for consistent handling of meetup leadership.
- ⚙️ Refactored `MeetupLeaderController` to use new leadership methods, improving reusability and maintainability.
- 👮‍♂️ Added `ValidNpub` validation rule for npub input standardization.
- 🧪 Added feature tests for leadership delegation and permissions.
- 🖼️ Implemented leader management UI in meetup edit page with flash messaging for actions.

app/Models/Meetup.php

@@ -8,6 +8,7 @@ use Illuminate\Database\Eloquent\Factories\HasFactory;
 use Illuminate\Database\Eloquent\Model;
 use Illuminate\Database\Eloquent\Relations\BelongsTo;
 use Illuminate\Database\Eloquent\Relations\HasMany;
+use Illuminate\Support\Collection;
 use Illuminate\Support\Facades\Cookie;
 use Illuminate\Support\Facades\Date;
 use Illuminate\Support\Facades\DB;
@@ -198,6 +199,45 @@ class Meetup extends Model implements HasMedia
             ->exists();
     }
 
+    /**
+     * Aktuelle Leader dieses Meetups (meetup_user.is_leader = true), Ersteller
+     * zuerst. Gemeinsame Quelle für REST-API (MeetupLeaderController) und das
+     * Portal-Frontend (meetups.edit).
+     *
+     * @return Collection<int, User>
+     */
+    public function leaders(): Collection
+    {
+        return $this->users()
+            ->wherePivot('is_leader', true)
+            ->get()
+            ->sortByDesc(fn (User $user): bool => $user->getKey() === $this->created_by)
+            ->values();
+    }
+
+    /**
+     * Nutzer zum Leader befördern (meetup_user.is_leader = true). Idempotent.
+     * Geteilt von REST-Controller, MCP-Tools und Portal-Frontend.
+     */
+    public function promoteLeader(User $user): void
+    {
+        $this->users()->syncWithoutDetaching([$user->getKey() => ['is_leader' => true]]);
+    }
+
+    /**
+     * Nutzer die Leader-Rolle entziehen (Demote; bleibt Mitglied). Der Ersteller
+     * des Meetups ist geschützt und wird nie entzogen (Domain-Invariante) —
+     * Aufrufer dürfen zusätzlich mit 403 antworten.
+     */
+    public function demoteLeader(User $user): void
+    {
+        if ($user->getKey() === $this->created_by) {
+            return;
+        }
+
+        $this->users()->updateExistingPivot($user->getKey(), ['is_leader' => false]);
+    }
+
     /**
      * Den Nutzer als Mitglied (nicht Leader) zu „Meine Meetups" hinzufügen.
      * Idempotent: ein bereits hinzugefügter Nutzer bleibt unverändert. Gibt