✨ 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.

tests/Feature/Console/PromoteExistingLeadersTest.php

@@ -0,0 +1,46 @@
+<?php
+
+use App\Models\City;
+use App\Models\Country;
+use App\Models\Meetup;
+use App\Models\User;
+use Illuminate\Support\Facades\DB;
+
+beforeEach(function () {
+    $country = Country::factory()->create(['code' => 'de']);
+    $this->city = City::factory()->create(['country_id' => $country->id]);
+});
+
+it('promotes all existing members to leaders', function () {
+    $meetup = Meetup::factory()->create(['city_id' => $this->city->id]);
+    $member = User::factory()->create();
+    $meetup->addMember($member); // is_leader = false
+
+    expect($meetup->isLeader($member))->toBeFalse();
+
+    $this->artisan('meetups:promote-existing-leaders')->assertSuccessful();
+
+    expect($meetup->fresh()->isLeader($member))->toBeTrue();
+});
+
+it('ensures the creator is a leader even for legacy meetups without a pivot row', function () {
+    $creator = User::factory()->create();
+    $meetup = Meetup::factory()->create(['city_id' => $this->city->id, 'created_by' => $creator->id]);
+    // Alt-Meetup simulieren: Ersteller-Pivot entfernen.
+    $meetup->users()->detach();
+    expect($meetup->hasMember($creator))->toBeFalse();
+
+    $this->artisan('meetups:promote-existing-leaders')->assertSuccessful();
+
+    expect($meetup->fresh()->isLeader($creator))->toBeTrue();
+});
+
+it('does not write on a dry run', function () {
+    $meetup = Meetup::factory()->create(['city_id' => $this->city->id]);
+    $member = User::factory()->create();
+    $meetup->addMember($member);
+
+    $this->artisan('meetups:promote-existing-leaders --dry-run')->assertSuccessful();
+
+    expect(DB::table('meetup_user')->where('user_id', $member->id)->value('is_leader'))->toBe(0);
+});