fix: prevent OIDC redirect URI construction from untrusted X-Forwarded-Host

The OIDC login route silently fell back to building the redirect URI from
X-Forwarded-Host/X-Forwarded-Proto when APP_URL was not configured. An
attacker could set X-Forwarded-Host: attacker.example.com to redirect the
authorization code to their own server after the user authenticates.

Remove the header-derived fallback entirely. If APP_URL is not set (via env
or the app_url DB setting), the OIDC login endpoint now returns a 500 error
rather than trusting attacker-controlled request headers. Document APP_URL
in .env.example as required for OIDC use.

server/.env.example

@@ -8,6 +8,8 @@ ALLOWED_ORIGINS=https://trek.example.com # Comma-separated origins for CORS and
 FORCE_HTTPS=false # Redirect HTTP → HTTPS behind a TLS proxy
 TRUST_PROXY=1 # Number of trusted proxies for X-Forwarded-For
 
+APP_URL=https://trek.example.com # Base URL of this instance — required when OIDC is enabled; must match the redirect URI registered with your IdP
+
 OIDC_ISSUER=https://auth.example.com # OpenID Connect provider URL
 OIDC_CLIENT_ID=trek # OpenID Connect client ID
 OIDC_CLIENT_SECRET=supersecret # OpenID Connect client secret