Add campaign UTM links and piha.li shortener (#4)

## Summary
- Port Monica campaign UTM + piha.li minting into always-on `core` (`shortener.py`, `campaign_utm.py`) so `email_sms` and `directmail` stay optional and never import each other.
- `utm_source` is a slug of `SITE_NAME` (override with `UTM_SOURCE`). Email gets an HTML `data-campaign-utm` link; SMS gets a plain URL; postcard QR only — no body inject.
- Live composer mints through login+CSRF `POST /portal/campaigns/short-link/` (registered only when an outreach app is installed). Empty `SHORTENER_*` falls back to the long UTM URL.

Reference: [monica_site PR #10](ai_ml_operations/monica_site#10)

Closes #3

## Test plan
- [x] `cd site && uv run python manage.py test` (125 tests)
- [ ] Email composer: type a name, confirm HTML link + `utm_campaign` updates, save draft
- [ ] SMS composer: plain `piha.li` (or long UTM if shortener unset) in the body
- [ ] Postcard composer: QR copies the tracked URL; campaign body has no `utm_source`
- [ ] Campaign report pages show the same panel
- [ ] With `FEATURE_EMAIL_SMS` and `FEATURE_DIRECT_MAIL` off, no extra nav and no `/portal/campaigns/short-link/` route

Reviewed-on: #4

site/client_site/urls.py

@@ -5,7 +5,7 @@ from django.contrib import admin
 from django.urls import include, path
 
 from contacts.views import address_suggest
-from core.views import healthz
+from core.views import campaign_short_link, healthz
 
 urlpatterns = [
     path("healthz/", healthz, name="healthz"),
@@ -24,6 +24,14 @@ if apps.is_installed("email_sms"):
     urlpatterns += [path("portal/messaging/", include("email_sms.urls"))]
 if apps.is_installed("directmail"):
     urlpatterns += [path("portal/direct-mail/", include("directmail.urls"))]
+if apps.is_installed("email_sms") or apps.is_installed("directmail"):
+    urlpatterns += [
+        path(
+            "portal/campaigns/short-link/",
+            campaign_short_link,
+            name="campaign_short_link",
+        ),
+    ]
 if apps.is_installed("blog"):
     urlpatterns += [
         path("blog/", include("blog.public_urls")),