Mint campaign UTM links through the piha.li shortener.

Closes #9.

Auto-insert tracked homepage links in campaign compose, register them with
url_shortening_service, and put short piha.li URLs in SMS, email hrefs, and
postcard QR codes when SHORTENER_* is configured.

site/messaging/README.md

@@ -35,6 +35,23 @@ webhook events match the correct recipient row. Invalid / missing header values
 no longer 500 the endpoint (SMTP2GO “Test this webhook” often sends a sample
 non-UUID).
 
+## Campaign tracked links (UTM + piha.li)
+
+Composer auto-inserts a homepage link:
+
+| Param | Value |
+|-------|--------|
+| `utm_source` | `monica` |
+| `utm_medium` | `email` / `sms` / `postcard` |
+| `utm_campaign` | hyphenated campaign name |
+
+When `SHORTENER_BASE_URL` + `SHORTENER_API_TOKEN` are set, the app POSTs that
+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`.
+
 SMS correlation uses `message_id` (SMS id), then `destination_number` phone
 fallback. Do **not** treat webhook `id` as the SMS id.