fix: Declare primary key type

https://laravel.com/docs/6.x/upgrade#eloquent-primary-key-type

app/Couple.php

@@ -2,8 +2,8 @@
 
 namespace App;
 
-use Ramsey\Uuid\Uuid;
 use Illuminate\Database\Eloquent\Model;
+use Ramsey\Uuid\Uuid;
 
 class Couple extends Model
 {
@@ -14,6 +14,13 @@ class Couple extends Model
      */
     public $incrementing = false;
 
+    /**
+     * The "type" of the primary key ID.
+     *
+     * @var string
+     */
+    protected $keyType = 'string';
+
     public function husband()
     {
         return $this->belongsTo(User::class);

app/User.php

@@ -19,6 +19,13 @@ class User extends Authenticatable
     public $incrementing = false;
 
     /**
+     * The "type" of the primary key ID.
+     *
+     * @var string
+     */
+    protected $keyType = 'string';
+
+    /**
      * The attributes that are mass assignable.
      *
      * @var array