Notify transfer owners when files enter grace period.

Send grace-entered and countdown emails to owners alongside recipient grace milestones so unpaid transfers get clear wallet top-up reminders before deletion.

Co-authored-by: Cursor <cursoragent@cursor.com>

database/migrations/2026_06_09_150000_add_owner_milestones_sent_to_transfers.php

@@ -0,0 +1,22 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    public function up(): void
+    {
+        Schema::table('transfers', function (Blueprint $table) {
+            $table->json('owner_milestones_sent')->nullable()->after('recipient_milestones_sent');
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('transfers', function (Blueprint $table) {
+            $table->dropColumn('owner_milestones_sent');
+        });
+    }
+};