fix: GD safety check + zlib-dev for robust Alpine build

- video_api.php: check function_exists('imagecreatefromstring') before calling GD
  — undefined function causes PHP fatal error even with @ suppressor, kills php-fpm
- Dockerfile: add zlib-dev (required for libpng on some Alpine versions)
- Dockerfile: verify GD loaded after install (build log confirmation)

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

backend/video_api.php

@@ -69,6 +69,12 @@ class VeoVideoAPI {
         [$targetW, $targetH] = $targets[$aspectRatio];
         $targetRatio = $targetW / $targetH;
 
+        // GD may not be installed — fatal errors from undefined functions can't be suppressed
+        if (!function_exists('imagecreatefromstring')) {
+            error_log("resizeImageForVeo: GD extension not available, skipping resize");
+            return $base64Data;
+        }
+
         $decoded = base64_decode($base64Data);
         if ($decoded === false) return $base64Data;
 

docker/Dockerfile

@@ -9,9 +9,10 @@ RUN npm run build
 
 # ── Stage 2: runtime (nginx + php-fpm) ────────────────────────────────────────
 FROM php:8.2-fpm-alpine
-RUN apk add --no-cache nginx supervisor libpng-dev libjpeg-turbo-dev \
+RUN apk add --no-cache nginx supervisor libpng-dev libjpeg-turbo-dev zlib-dev \
     && docker-php-ext-configure gd --with-jpeg \
-    && docker-php-ext-install -j$(nproc) gd
+    && docker-php-ext-install -j$(nproc) gd \
+    && php -m | grep -i gd && echo "GD installed OK" || echo "GD missing — resize disabled"
 
 # Composer
 COPY --from=composer:2 /usr/bin/composer /usr/bin/composer