Add guided installer for SaaS application setup

- Updated README.md to include instructions for guided installation on webspace without shell access.
- Enhanced installationshandbuch.md with detailed steps for using the guided installer.
- Modified saas-app/README.md to summarize the installation process using the new installer.
- Generated SQL migration bundle now includes timestamp for better tracking.
- Introduced public/install/index.php as the entry point for the guided installer.
- Updated public/index.php to reflect changes in installation steps and added links to the installer.
- Refactored migration bundle generation and SQL execution scripts for improved error handling and modularity.
- Implemented session-based CSRF protection in the installer.
- Added form handling for environment variable configuration and migration execution in the installer.
- Created a user-friendly HTML interface for the installer with status messages and error handling.

README.md

@@ -29,6 +29,9 @@ Archivbestand erhalten.
    Setup, Hosting und Betriebsablauf.
 3. Arbeite anschliessend in [saas-app/README.md](saas-app/README.md) weiter.
 
+Auf Webspace ohne Shell ist der bevorzugte Einstieg die gefuehrte Installation
+unter `saas-app/public/install/`.
+
 ## Hilfsskripte
 
 - `scripts/check-prerequisites.php` prueft lokale Voraussetzungen.