✨ 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/Meetup.php

@@ -185,6 +185,19 @@ class Meetup extends Model implements HasMedia
         return $this->users()->whereKey($user->id)->exists();
     }
 
+    /**
+     * Ist der Nutzer Leader dieses Meetups (meetup_user.is_leader = true)?
+     * Nur Leader (und der Ersteller/Super-Admin) dürfen Stammdaten bearbeiten
+     * und weitere Leader einsetzen/entziehen.
+     */
+    public function isLeader(User $user): bool
+    {
+        return $this->users()
+            ->whereKey($user->id)
+            ->wherePivot('is_leader', true)
+            ->exists();
+    }
+
     /**
      * Den Nutzer als Mitglied (nicht Leader) zu „Meine Meetups" hinzufügen.
      * Idempotent: ein bereits hinzugefügter Nutzer bleibt unverändert. Gibt
@@ -233,6 +246,32 @@ class Meetup extends Model implements HasMedia
         });
     }
 
+    /**
+     * Meetups, die der Nutzer als Leader führt (meetup_user.is_leader = true).
+     * Maßgeblich dafür, wer Stammdaten UND Termine bearbeiten darf.
+     */
+    public function scopeLedBy(Builder $query, int $userId): void
+    {
+        $query->whereHas('users', fn (Builder $user) => $user->whereKey($userId)->wherePivot('is_leader', true));
+    }
+
+    /**
+     * Führt der eingeloggte Nutzer dieses Meetup als Leader? Steuert die
+     * Sichtbarkeit der Bearbeiten-/Termin-Affordances im Portal-Frontend
+     * (Gegenstück zu {@see belongsToMe()}, aber leader- statt mitgliedschafts-
+     * basiert).
+     */
+    protected function leadByMe(): Attribute
+    {
+        return Attribute::make(
+            get: fn (): bool => auth()->check() && DB::table('meetup_user')
+                ->where('meetup_id', $this->id)
+                ->where('user_id', auth()->id())
+                ->where('is_leader', true)
+                ->exists()
+        );
+    }
+
     public function city(): BelongsTo
     {
         return $this->belongsTo(City::class);

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()