feat(payment): implement secure 1:1 Polar customer binding system

- Add polar_cust_id column to users table for direct customer mapping
  - Rewrite PolarProvider customer logic to use stored customer IDs
  - Eliminate email-based customer lookup to prevent cross-user contamination
  - Implement self-healing mechanism for invalid customer IDs
  - Maintain external_id binding for Polar's reference system
  - Add comprehensive logging for customer lookup operations

database/migrations/2025_12_04_204710_add_polar_cust_id_to_users_table.php

@@ -0,0 +1,29 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    /**
+     * Run the migrations.
+     */
+    public function up(): void
+    {
+        Schema::table('users', function (Blueprint $table) {
+            // Add polar_cust_id column after 'id' for 1:1 Polar customer binding
+            $table->string('polar_cust_id')->nullable()->after('id');
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        Schema::table('users', function (Blueprint $table) {
+            $table->dropColumn('polar_cust_id');
+        });
+    }
+};