✨ Add addToMine functionality to Meetups API for adding meetups to a user's "My Meetups" list

- 🔒 Introduce `addToMine` policy for authenticated users to add existing meetups.
- ✏️ Add `addToMine` method in `MeetupController` with idempotent handling.
- ✨ Include `addMember` utility in `Meetup` model for managing pivot relationships.
- 🛠️ Refactor `AddMeetupToMineTool` to use `addMember` for consistency.
- 🧪 Add feature tests for `addToMine`, covering idempotency, permissions, and unknown slugs.
- 🌐 Register `addToMine` route in API and link it to `MeetupController`.

app/Models/Meetup.php

@@ -185,6 +185,23 @@ class Meetup extends Model implements HasMedia
         return $this->users()->whereKey($user->id)->exists();
     }
 
+    /**
+     * Den Nutzer als Mitglied (nicht Leader) zu „Meine Meetups" hinzufügen.
+     * Idempotent: ein bereits hinzugefügter Nutzer bleibt unverändert. Gibt
+     * true zurück, wenn neu hinzugefügt (false = war bereits Mitglied).
+     * Geteilt von REST-Controller (addToMine) und MCP-Tool (AddMeetupToMineTool).
+     */
+    public function addMember(User $user): bool
+    {
+        $wasMember = $this->hasMember($user);
+
+        $this->users()->syncWithoutDetaching([
+            $user->getKey() => ['is_leader' => false],
+        ]);
+
+        return ! $wasMember;
+    }
+
     /**
      * RateLimiter-Key für Meetup-Stammdaten-Updates über das Portal-Frontend.
      */