Mint campaign UTM links through the piha.li shortener (#10)

## Summary
Closes #9.

- Campaign composer auto-inserts a tracked homepage link (`utm_source=monica`, `utm_medium` = channel, `utm_campaign` = slug of the name) for email, SMS, and postcard QR — no manual UTM paste.
- When `SHORTENER_BASE_URL` + `SHORTENER_API_TOKEN` are set, the app mints that long HTTPS URL via `POST /api/links/` on the shortener **API host** and puts the returned `piha.li` / `beta.piha.li` short URL in SMS, email hrefs, and QR codes. Empty env (local) falls back to the long UTM URL.
- Live composer resolves shorts through a portal JSON endpoint (login + CSRF). Browser never calls the shortener.

## Secrets (control node, not git)

**monica_site** (`~/Documents/secrets/monica_site/`):
```
# prod
SHORTENER_BASE_URL=https://shortener.aimloperations.com
SHORTENER_API_TOKEN=monica:<secret>

# beta
SHORTENER_BASE_URL=https://shortener-beta.aimloperations.com
SHORTENER_API_TOKEN=monica:<beta-secret>
```

**url_shortening_service** (same secret, named token):
```
SHORTENER_API_TOKENS=monica:<secret>
SHORT_ALLOWED_HOSTS=mkdrealtor.com,aimloperations.com
```
Prod public short host: `piha.li`. Beta: `beta.piha.li`. Generate with `python -c "import secrets; print(secrets.token_urlsafe(32))"`.

Template port: westfarn/web_django_template#3

## Test plan
- [ ] `cd site && uv run python manage.py test messaging.tests.CampaignUtmLinkTests`
- [ ] Composer: type a campaign name — email gets an HTML link, SMS gets a URL, postcard shows a QR
- [ ] With shortener env set: SMS/QR show `piha.li` (or `beta.piha.li`); without it, long UTM URL still works
- [ ] Copy/download QR into postcard designer
- [ ] Secret files have `SHORTENER_*` on both caller and operator sides before beta/prod deploy

Reviewed-on: #10

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.