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

PROXY_FIX_SETUP.md

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+# ProxyFix Middleware Setup - DigiServer v2
+
+## Overview
+ProxyFix middleware is now properly configured in the Flask app to handle reverse proxy headers from Nginx (or Caddy). This ensures correct handling of:
+- **X-Real-IP**: Client's real IP address
+- **X-Forwarded-For**: List of IPs in the proxy chain
+- **X-Forwarded-Proto**: Original protocol (http/https)
+- **X-Forwarded-Host**: Original hostname
+
+## Configuration Details
+
+### Flask App (app/app.py)
+```python
+from werkzeug.middleware.proxy_fix import ProxyFix
+
+app.wsgi_app = ProxyFix(app.wsgi_app, x_for=1, x_proto=1, x_host=1, x_port=1)
+```
+
+**Parameters:**
+- `x_for=1`: Trust one proxy for X-Forwarded-For header
+- `x_proto=1`: Trust proxy for X-Forwarded-Proto header
+- `x_host=1`: Trust proxy for X-Forwarded-Host header
+- `x_port=1`: Trust proxy for X-Forwarded-Port header
+
+### Config Settings (app/config.py)
+
+```python
+# Reverse proxy trust (for Nginx/Caddy with ProxyFix middleware)
+TRUSTED_PROXIES = os.getenv('TRUSTED_PROXIES', '127.0.0.1,10.0.0.0/8,172.16.0.0/12,192.168.0.0/16')
+PREFERRED_URL_SCHEME = os.getenv('PREFERRED_URL_SCHEME', 'https')
+```
+
+## Testing ProxyFix
+
+### 1. Test Real Client IP
+```bash
+docker exec digiserver-app flask shell
+>>> from flask import request
+>>> request.remote_addr  # Should show client IP
+```
+
+### 2. Test URL Scheme
+```bash
+docker exec digiserver-app flask shell
+>>> from flask import url_for
+>>> url_for('auth.login', _external=True)  # Should use https://
+```
+
+## Verification Checklist
+
+- [x] ProxyFix imported in app.py
+- [x] app.wsgi_app wrapped with ProxyFix
+- [x] TRUSTED_PROXIES configured
+- [x] PREFERRED_URL_SCHEME set to 'https'
+- [x] SESSION_COOKIE_SECURE=True in ProductionConfig
+- [x] Nginx headers configured correctly