[FEATURE] Trusted proxies should be configurable.

[DanielOliver]

The BardView5 Server application uses Gin. Currently when running behind a proxy, Gin trusts all proxies by default which is unsafe.

For the convenience and security of anyone who is willing to do the extra configuration when hosting BV5 themselves, the trusted proxies using Gin's SetTrustedProxies should be configurable from environment variables. If none set, default of trusting all should be the fallback.