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/links/auth.py

@@ -1,4 +1,4 @@
-"""Bearer token auth for /api/. The lock that keeps a public API host closed."""
+"""Bearer token auth for /api/. The lock that keeps minting closed on the public host."""
 
 from __future__ import annotations