document compatibility workarounds

app/Providers/AppServiceProvider.php

@@ -20,7 +20,9 @@ class AppServiceProvider extends ServiceProvider
             return new CustomValidator($translator, $data, $rules, $messages);
         });
 
-        // Set default string length in order to run on older versions of MySQL
+        // TODO(compat): This limits strings to 191 characters for older MySQL
+        // utf8mb4 index limits. Remove once every supported database handles
+        // indexed 255-character utf8mb4 strings; verify fresh migrations first.
         Schema::defaultStringLength(191);
     }