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/settings/base.py

@@ -2,6 +2,7 @@
 
 import json
 import os
+import sys
 from pathlib import Path
 from urllib.parse import urlparse
 
@@ -364,6 +365,20 @@ NOMINATIM_USER_AGENT = env(
 )
 NOMINATIM_API_KEY = env("NOMINATIM_API_KEY", "")
 
+# --- URL shortener (piha.li) ---
+# Call the API host, never the public short domain. Empty = skip minting
+# (composer falls back to the long UTM URL). Token format: name:secret.
+# Required in beta/prod when FEATURE_EMAIL_SMS or FEATURE_DIRECT_MAIL is on.
+SHORTENER_BASE_URL = env("SHORTENER_BASE_URL", "")
+SHORTENER_API_TOKEN = env("SHORTENER_API_TOKEN", "")
+SHORTENER_TIMEOUT_SECONDS = int(env("SHORTENER_TIMEOUT_SECONDS", "10") or "10")
+# Optional override for utm_source. Empty → slug of SITE_NAME.
+UTM_SOURCE = env("UTM_SOURCE", "")
+# manage.py test must not mint against a live API (env often has prod tokens).
+if "test" in sys.argv:
+    SHORTENER_BASE_URL = ""
+    SHORTENER_API_TOKEN = ""
+
 # --- Tianji analytics (pageviews + events; notice banner, not opt-in) ---
 TIANJI_ENABLED = env_bool("TIANJI_ENABLED", not DEBUG)
 TIANJI_TRACKER_URL = env(