Issue #205 — Django i18n foundation (da/en)

Implemented acceptance checks

Django i18n setup for en + da with en fallback
- LANGUAGE_CODE set to en.
- LANGUAGES = [('en', 'English'), ('da', 'Danish')].
- LocaleMiddleware enabled in middleware chain.
Shared-key resolver/adapter (no ad hoc backend mapping)
- Backend error responses now resolve from shared catalog keys in shared/i18n/lobby.json.
- lobby.i18n.api_error() accepts a shared key and resolves locale-specific text.
Representative API flow documented with key/locale behavior
- POST /lobby/join with empty code returns:
  - error_code: "session_code_required"
  - localized error
  - resolved locale
Missing key handling deterministic and loggable
- resolve_error_message() returns key string when key/translation is missing.
- Warning is logged (lobby.i18n logger) for missing key/translation.

POST /lobby/join with empty code and header Accept-Language: da

{
  "error": "Sessionskode er påkrævet",
  "error_code": "session_code_required",
  "locale": "da"
}

If locale is unsupported (e.g. fr), response uses locale: "en" and English message.