fix(docker): upgrade to PHP 8.4 to match composer.lock requirements

symfony/translation in the lock file requires PHP >= 8.4.1, causing
container crash on startup with the 8.3 image.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Dockerfile

@@ -21,7 +21,7 @@ COPY . .
 RUN npm run build
 
 # Stage 3: Production image
-FROM php:8.3-fpm-alpine
+FROM php:8.4-fpm-alpine
 
 # Install system dependencies
 RUN apk add --no-cache \