Mobile auth: native login + register via central identity API.

Login no longer uses the OAuth password grant (which wasn't enabled, the cause
of login failures). Both login and the new register endpoint proxy to
auth.ladill.com /api/identity/auth/* (shared IDENTITY_API_KEY_MINI), provision
the local mirror, and issue a Sanctum token — one Ladill identity across web
and app.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

config/services.php

@@ -26,6 +26,14 @@ return [
         'redirect' => rtrim((string) env('APP_URL', 'https://mini.ladill.com'), '/').'/sso/callback',
     ],
 
+    // Central Ladill identity API (auth.ladill.com /api/identity/auth/*). The
+    // mobile app's native login/register proxy through here, gated by the
+    // shared first-party service key (config/identity.php on the monolith).
+    'ladill_identity' => [
+        'url' => 'https://'.config('app.auth_domain'),
+        'key' => env('IDENTITY_API_KEY_MINI'),
+    ],
+
     'ladill_webmail' => [
         'url' => env('LADILL_WEBMAIL_URL', 'https://mail.ladill.com'),
     ],