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

.env.example

@@ -85,6 +85,16 @@ 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).
+# Required in beta/prod when FEATURE_EMAIL_SMS or FEATURE_DIRECT_MAIL is on.
+SHORTENER_BASE_URL=
+SHORTENER_API_TOKEN=
+# UTM_SOURCE=  # optional override; default is a slug of SITE_NAME
+# Local shortener compose:
+# SHORTENER_BASE_URL=http://127.0.0.1:8005
+# SHORTENER_API_TOKEN=acme:dev-only-token
+
 # Nominatim (LAN) — address autocomplete via Django /api/address-suggest/
 NOMINATIM_BASE_URL=http://10.0.0.128:8089
 NOMINATIM_TIMEOUT_SECONDS=8