build: optimize Docker setup for Caddy SPA and improve runtime stability

Updated Dockerfile to streamline the hybrid build process and optimize artifact integration. Modified Caddyfile for better routing logic, enhanced security headers, Prometheus metrics, and improved API proxy handling. Adjusted `dc-gui.yaml` for container stability and simplified runtime configuration for `apiBaseUrl`. Expanded documentation with troubleshooting and session logs.

config/docker/caddy/web-app/Dockerfile

@@ -1,7 +1,7 @@
 # syntax=docker/dockerfile:1.8
 # ===================================================================
-# Dockerfile for Meldestelle Web-App (Pre-built Artifacts)
-# Version: 3.1.0 - Local Build Injection
+# Dockerfile for Meldestelle Web-App (Hybrid Build)
+# Version: 3.2.0 - Optimized & Cleaned
 # ===================================================================
 
 # === GLOBAL ARGS ===
@@ -31,6 +31,7 @@ COPY config/docker/caddy/web-app/config.json /usr/share/caddy/config.json
 COPY frontend/shells/meldestelle-portal/build/dist/js/productionExecutable/ /usr/share/caddy/
 
 # Ensure favicon exists (fallback)
+# Using the shared asset from existing config structure
 COPY config/docker/nginx/web-app/favicon.svg /usr/share/caddy/favicon.svg
 
 EXPOSE 4000