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/README.md

@@ -50,7 +50,7 @@ long HTTPS URL to `url_shortening_service` (`POST /api/links/`, Bearer
 `monica:<secret>`) and puts `short_url` (`https://piha.li/<code>`) in SMS,
 email hrefs, and postcard QR codes. Empty env → long UTM URL (local default).
 
-Mint against the **API host**, not `piha.li`. See `.env.example` / `.env.prod.example`.
+Mint against the short host (`https://piha.lc` / `https://beta.piha.li`). See `.env.example` / `.env.prod.example`.
 
 SMS correlation uses `message_id` (SMS id), then `destination_number` phone
 fallback. Do **not** treat webhook `id` as the SMS id.