Add standalone /publiser admin application

Implement a minimal, dependency-free publishing tool to manage news
and articles. The app includes:

- A Markdown-based block editor with rich-text capabilities.
- Image upload and resizing using GD.
- Support for drafts, scheduled publishing, and a trash system.
- Integrated Basic Auth protection via PHP and .htpasswd.
- A global plugin for automatic execution of scheduled posts.

The tool reuses core site logic in custom/plugins/publiser-lib.php to
ensure consistency between the editor and the live site.

Containerfile

@@ -3,6 +3,13 @@ FROM php:8.4.14-apache
 # Enable Apache modules and custom config as root during build
 RUN a2enmod rewrite headers
 
+# GD extension - used by /publiser to resize/recompress uploaded images
+RUN apt-get update \
+    && apt-get install -y --no-install-recommends libjpeg62-turbo-dev libpng-dev libwebp-dev \
+    && docker-php-ext-configure gd --with-jpeg --with-webp \
+    && docker-php-ext-install gd \
+    && rm -rf /var/lib/apt/lists/*
+
 COPY apache.conf /etc/apache2/conf-available/custom.conf
 RUN a2enconf custom