engine: remove dead UI/stats/data handlers now owned by the front

With --single-process gone, the engine only ever runs behind the front, which
serves these before its catch-all — so the engine copies were dead code. Removed
from codai/admin/routes.py:
  - api_status (GET /admin/api/status) — front builds status from its own state
  - api_list_tokens/api_create_token/api_delete_token — front admin_data (auth.json)
  - api_create_user/api_delete_user — front admin_data (auth.json)
  - api_get_settings (GET) — front serves via the shared build_settings_dict

Kept: build_settings_dict (imported by the front), api_save_settings (POST — the
engine still persists/applies config), api_list_models, and the auth POSTs
(login/logout/change-password) the front still proxies.

The engine now does generation + model management only; the front owns pages,
stats and these data reads. Verified routes.py imports and compiles.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011DDv7BchtZQWsnPG6Jm49m