feat: add trust_forwarded_proto option for SSL redirect handling in reverse proxy scenarios

When Nginx is behind another proxy server (like CloudFlare or AWS ALB), the force-SSL
feature can cause redirect loops because Nginx sees the connection as plain HTTP
while SSL is already handled upstream. This adds a new boolean option to trust
the X-Forwarded-Proto header from upstream proxies.

Changes:
- Add `trust_forwarded_proto` column to proxy_host table (migration)
- Update model and API schema to support the new boolean field
- Modify force-ssl Nginx template to check X-Forwarded-Proto/X-Forwarded-Scheme
- Add map directives in nginx.conf to validate and sanitize forwarded headers
- Add advanced option toggle in frontend UI with i18n support (EN/ZH)
- Set proxy headers from validated map variables instead of $scheme

This allows administrators to control SSL redirect behavior when Nginx is deployed
behind a TLS-terminating proxy.

backend/templates/_forced_ssl.conf

@@ -1,6 +1,11 @@
 {% if certificate and certificate_id > 0 -%}
 {% if ssl_forced == 1 or ssl_forced == true %}
     # Force SSL
+    {% if trust_forwarded_proto == true %}
+    set $trust_forwarded_proto "T";
+    {% else %}
+    set $trust_forwarded_proto "F";
+    {% endif %}
     include conf.d/include/force-ssl.conf;
 {% endif %}
 {% endif %}