OpenPlex 0.29.0

feat(i18n): translatable API messages with values

Messages that carry values (quotas, overlapping names, invalid JSON or
ids, PHP settings, allow-lists, size limits, e-mail addresses, captcha
keys...) are now built with internal/msg: a fixed English template with
{name} placeholders and its values. Errors carry the template and the
values next to the English message, and the dashboard translates the
template before filling it in. A test fails when a template has no
Spanish or Catalan entry in web/locales/*/server.json. Unique-constraint
conflicts answer a plain "already exists" instead of the SQLite text.