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/static/css/portal.css

@@ -690,3 +690,39 @@ body.portal {
   padding: 8px 10px; border: 1px solid var(--monica-border); font: inherit; font-size: 13px; background: #fff;
 }
 .empty-state { padding: 24px; color: var(--monica-muted); font-size: 14px; }
+
+.utm-panel {
+  margin-top: 4px;
+  padding-top: 4px;
+}
+.utm-url {
+  display: block;
+  font-size: 12px;
+  line-height: 1.45;
+  word-break: break-all;
+  background: #f8fafc;
+  border: 1px solid var(--monica-border);
+  padding: 8px 10px;
+  color: #0f172a;
+}
+.utm-actions {
+  display: flex;
+  gap: 8px;
+  flex-wrap: wrap;
+  margin-top: 8px;
+}
+.utm-qr {
+  display: flex;
+  gap: 16px;
+  align-items: flex-start;
+  flex-wrap: wrap;
+  margin-top: 12px;
+}
+.utm-qr canvas {
+  display: block;
+  border: 1px solid var(--monica-border);
+  background: #fff;
+}
+.utm-qr-actions { min-width: 160px; }
+.utm-qr-actions .btn { margin: 0 8px 8px 0; }
+#preview-body a { color: var(--monica-primary); }