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/Models/AiProvider.php

@@ -11,6 +11,14 @@ use Illuminate\Database\Eloquent\Model;
 #[Hidden(['api_key'])]
 class AiProvider extends Model
 {
+    public static function resolveActiveDefault(): ?self
+    {
+        return static::query()
+            ->where('is_active', true)
+            ->orderByDesc('is_default')
+            ->first();
+    }
+
     public function resolvedBaseUrl(): string
     {
         return $this->base_url ?: $this->driver->defaultBaseUrl();