✨ 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/Rules/ValidNpub.php

@@ -0,0 +1,30 @@
+<?php
+
+namespace App\Rules;
+
+use Closure;
+use Illuminate\Contracts\Validation\ValidationRule;
+use swentel\nostr\Key\Key as NostrKey;
+
+/**
+ * Validiert einen Nostr-npub (bech32-kodierter öffentlicher Schlüssel).
+ * Geteilt von der REST-API (StoreMeetupLeaderRequest) und der Leader-
+ * Verwaltung im Portal-Frontend, damit beide Wege dieselbe Prüfung nutzen.
+ */
+class ValidNpub implements ValidationRule
+{
+    public function validate(string $attribute, mixed $value, Closure $fail): void
+    {
+        if (! is_string($value) || ! str_starts_with($value, 'npub1')) {
+            $fail(__('Das ist kein gültiger npub.'));
+
+            return;
+        }
+
+        try {
+            (new NostrKey)->convertToHex($value);
+        } catch (\Throwable) {
+            $fail(__('Das ist kein gültiger npub.'));
+        }
+    }
+}