feat: Docker Compose setup untuk development & production

- docker/php/Dockerfile: PHP 8.4-FPM + GD + imagick (PECL) + semua extension Laravel
- docker/php/php.ini: upload 20MB, memory 512MB, opcache, Asia/Kuala_Lumpur
- docker/php/php-dev.ini: validate_timestamps=1, display_errors=On (dev)
- docker/nginx/default.conf: gzip, security headers, static asset caching
- docker/entrypoint.sh: tunggu MySQL → migrate → seed AdminSeeder → cache (prod)
- docker-compose.yml: dev stack — port 8003, DB host 33060, queue worker
- docker-compose.prod.yml: production overrides — storage volume, no DB port exposed
- .env.docker: template env untuk Docker (DB_HOST=db)
- .dockerignore: exclude node_modules, vendor, .env, logs

fix: testGenerate try/catch kembalikan JSON error (bukan HTML 500)
fix: loadPreview() semak r.ok, tunjuk error alert, loading spinner

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/Http/Controllers/Admin/CertificateTemplateController.php

@@ -94,7 +94,7 @@ class CertificateTemplateController extends Controller
         ]);
     }
 
-    public function testGenerate(Request $request, Program $program, CertificateService $service): Response
+    public function testGenerate(Request $request, Program $program, CertificateService $service): \Illuminate\Http\Response|\Illuminate\Http\JsonResponse
     {
         $template = $program->certificateTemplate;
         abort_if(! $template, 404);
@@ -102,7 +102,11 @@ class CertificateTemplateController extends Controller
         $sampleName = $request->input('sample_name', 'NAMA PESERTA CONTOH');
         $sampleNo   = $request->input('sample_no', 'ECT/2025/0001');
 
-        $imageData = $service->generatePreview($template, $sampleName, $sampleNo);
+        try {
+            $imageData = $service->generatePreview($template, $sampleName, $sampleNo);
+        } catch (\Throwable $e) {
+            return response()->json(['error' => $e->getMessage()], 500);
+        }
 
         return response($imageData, 200, [
             'Content-Type'        => 'image/jpeg',