feat(api): add AI text-to-deck generation, wallet deposit structure, and profile/notification endpoints

Fills the gaps between the admin panel screens and the backend: a full
free-text + slide-count + language -> AI-generated presentation flow with
new slide-cloning logic in TemplateFillService, a "my presentations" list
endpoint, a swappable payment-gateway structure for wallet top-ups (fake
driver for now), and avatar upload / public-profile toggle / account
deletion / password change / notification preference endpoints.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/Contracts/PaymentGateway.php

@@ -0,0 +1,21 @@
+<?php
+
+namespace App\Contracts;
+
+use App\Models\WalletDeposit;
+
+interface PaymentGateway
+{
+    /**
+     * Start a payment for the given deposit and return the URL the user should be
+     * redirected to in order to complete it.
+     */
+    public function initiate(WalletDeposit $deposit): string;
+
+    /**
+     * Verify a gateway callback for the given deposit.
+     *
+     * @param  array<string, mixed>  $callbackPayload
+     */
+    public function verify(WalletDeposit $deposit, array $callbackPayload): bool;
+}