Add OIDC login support for external identity providers

Adds "Login with <provider>" as an alternative login method using the
jumbojett/openid-connect-php library. OIDC users must already exist in
the database (matched by email). Configurable via OIDC_PROVIDER_URL,
OIDC_CLIENT_ID, OIDC_CLIENT_SECRET, and OIDC_PROVIDER_NAME constants.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

models/Router.php

@@ -20,6 +20,7 @@ class Router
 			'edituser' => 'EditUser',
 			'login' => 'Login',
 			'logout' => 'Logout',
+			'oidclogin' => 'OIDCLogin',
 			'managealbums' => 'ManageAlbums',
 			'manageassets' => 'ManageAssets',
 			'manageerrors' => 'ManageErrors',