Expose public auth capabilities endpoint for mobile SSO detection.

Returns whether Authentik and signups are enabled so the app can show the right
sign-in options per instance at runtime.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/app/api/auth/capabilities/route.ts

@@ -0,0 +1,10 @@
+import { NextResponse } from "next/server";
+
+import { env } from "~/env";
+
+export function GET() {
+  return NextResponse.json({
+    authentik: env.NEXT_PUBLIC_AUTHENTIK_ENABLED === true,
+    signupsDisabled: env.DISABLE_SIGNUPS === true,
+  });
+}