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

.env.example

@@ -79,8 +79,8 @@ SOCIAL_TOKEN_ENCRYPTION_KEY=
 OLLAMA_BASE_URL=http://10.0.0.128:11434
 OLLAMA_MODEL=llama3.2
 
-# URL shortener (piha.li). Empty locally = long UTM URLs in campaigns.
-# Call the API host, never https://piha.li (that host only 302s).
+# URL shortener (piha.lc). Empty locally = long UTM URLs in campaigns.
+# Call the short host: POST /api/links/ (Bearer) and recipients GET /<code>.
 SHORTENER_BASE_URL=
 SHORTENER_API_TOKEN=
 # Local shortener compose: