Serve /api/ on piha.lc / beta.piha.li (#8)

## Summary
- Serve `/api/` on `piha.lc` / `beta.piha.li` (Bearer still required).
- Drop `shortener.aimloperations.com` / `shortener-beta.aimloperations.com` from env examples and caller docs.
- `/admin/` stays 404 on the public host.

Closes #7.

## Test plan
- [ ] `POST https://beta.piha.li/api/links/` with valid Bearer → 201
- [ ] Same without Bearer → 401
- [ ] `GET https://beta.piha.li/<code>` still 302
- [ ] `/admin/` on beta.piha.li → 404
- [ ] Unit tests: `cd site && uv run python manage.py test`

Reviewed-on: #8

site/shortener/settings/base.py

@@ -184,7 +184,7 @@ SHORT_API_HOSTS = env_list(
     "SHORT_API_HOSTS",
     "localhost,127.0.0.1,0.0.0.0,testserver,web,url-shortener",
 )
-# Django admin — local/dev only. Never put the public API hostname here.
+# Django admin — local/dev only. Never put the public short hostname here.
 SHORT_ADMIN_HOSTS = env_list("SHORT_ADMIN_HOSTS", "localhost,127.0.0.1")
 SHORTENER_API_TOKENS = parse_api_tokens(env("SHORTENER_API_TOKENS", "") or "")
 SHORT_ALLOWED_HOSTS = env_list(