Point the mailbox model at the UUID trait the repo already has

Seventeen models use App\Models\Concerns\HasUuid, and R11 is the reason:
URLs address records by UUID, not by integer primary key. The plan told
the implementer to hand-roll booted() instead, losing getRouteKeyName()
with it. Authoring error, corrected before the remaining tasks copy it.

docs/superpowers/plans/2026-07-27-mailboxes.md

@@ -359,10 +359,10 @@ return new class extends Migration
 
 namespace App\Models;
 
+use App\Models\Concerns\HasUuid;
 use App\Services\Secrets\SecretCipher;
 use Illuminate\Database\Eloquent\Factories\HasFactory;
 use Illuminate\Database\Eloquent\Model;
-use Illuminate\Support\Str;
 
 /**
  * One sending address.
@@ -373,7 +373,10 @@ use Illuminate\Support\Str;
  */
 class Mailbox extends Model
 {
-    use HasFactory;
+    // HasUuid assigns the uuid on create AND makes it the route key — R11:
+    // URLs address records by UUID, not by integer primary key. Seventeen
+    // models already use it; do not hand-roll a booted() replacement.
+    use HasFactory, HasUuid;
 
     protected $fillable = [
         'key', 'address', 'display_name', 'username',
@@ -389,13 +392,6 @@ class Mailbox extends Model
         ];
     }
 
-    protected static function booted(): void
-    {
-        static::creating(function (self $box) {
-            $box->uuid ??= (string) Str::uuid();
-        });
-    }
-
     public static function findByKey(string $key): ?self
     {
         return static::query()->where('key', $key)->first();