Mint short links on piha.lc / beta.piha.li (#12)

## Summary
- Point `SHORTENER_BASE_URL` at `https://piha.lc` (prod) and `https://beta.piha.li` (beta).
- Stop documenting `shortener.aimloperations.com` as the mint host.

Closes #11.

Depends on url_shortening_service serving `/api/` on the short host.

## Test plan
- [ ] Set `SHORTENER_BASE_URL=https://beta.piha.li` and matching token in beta env
- [ ] Compose a campaign SMS and confirm mint hits beta.piha.li (not shortener-beta)
- [ ] Body contains `https://beta.piha.li/<code>`

Reviewed-on: #12

site/messaging/shortener.py

@@ -1,6 +1,6 @@
-"""Server-to-server client for url_shortening_service (piha.li).
+"""Server-to-server client for url_shortening_service (piha.lc).
 
-Call the API host (`SHORTENER_BASE_URL`), never the public short domain.
+Call SHORTENER_BASE_URL (https://piha.lc / https://beta.piha.li).
 Auth: Authorization: Bearer <SHORTENER_API_TOKEN> (name:secret).
 """