Fix: fallback to source install if dist download fails

Handles transient GitHub CDN 400 errors during Docker build.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Dockerfile

@@ -28,7 +28,12 @@ RUN composer install \
     --no-interaction \
     --no-scripts \
     --no-autoloader \
-    --prefer-dist
+    --prefer-dist || composer install \
+    --no-dev \
+    --no-interaction \
+    --no-scripts \
+    --no-autoloader \
+    --prefer-source
 
 COPY . .
 RUN composer dump-autoload --optimize --no-dev