feat: complete nginx migration from caddy

- Replace Caddy reverse proxy with Nginx (nginx:alpine)
- Add nginx.conf with HTTP/HTTPS, gzip, and proxy settings
- Add nginx-custom-domains.conf template for custom domains
- Update docker-compose.yml to use Nginx service
- Add ProxyFix middleware to Flask app for proper header handling
- Create nginx_config_reader.py utility to read Nginx configuration
- Update admin blueprint to display Nginx status in https_config page
- Add Nginx configuration display to https_config.html template
- Generate self-signed SSL certificates for localhost
- Add utility scripts: generate_nginx_certs.sh
- Add documentation: NGINX_SETUP_QUICK.md, PROXY_FIX_SETUP.md
- All containers now running, HTTPS working, HTTP redirects to HTTPS
- Session cookies marked as Secure
- Security headers properly configured

nginx-custom-domains.conf

@@ -0,0 +1,21 @@
+# Nginx configuration for custom HTTPS domains
+# This file will be dynamically generated based on HTTPSConfig database entries
+# Include this in your nginx.conf with: include /etc/nginx/conf.d/custom-domains.conf;
+
+# Example entry for custom domain:
+# server {
+#     listen 443 ssl http2;
+#     listen [::]:443 ssl http2;
+#     server_name digiserver.example.com;
+#
+#     ssl_certificate /etc/nginx/ssl/custom/cert.pem;
+#     ssl_certificate_key /etc/nginx/ssl/custom/key.pem;
+#
+#     location / {
+#         proxy_pass http://digiserver_app;
+#         proxy_set_header Host $host;
+#         proxy_set_header X-Real-IP $remote_addr;
+#         proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+#         proxy_set_header X-Forwarded-Proto $scheme;
+#     }
+# }