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/directmail/views.py

@@ -27,6 +27,7 @@ from directmail.providers.postcard.pcm import (
     list_designs,
 )
 from contacts.consent import opted_in_contacts
+from core.campaign_utm import utm_panel_context
 from core.scheduling import parse_scheduled_for
 from directmail.services import (
     create_campaign_draft,
@@ -58,8 +59,6 @@ _MAX_IMAGE_BYTES = 5 * 1024 * 1024
 def _audience_choices() -> list[tuple[str, str]]:
     """Labeled audience options with live opted-in counts."""
     rows = [
-        (Campaign.Audience.POSTCARD_OPT_IN, Channel.EMAIL, "email"),
-        (Campaign.Audience.SMS_OPT_IN, Channel.SMS, "SMS"),
         (Campaign.Audience.POSTCARD_OPT_IN, Channel.POSTCARD, "postcard"),
     ]
     choices = []
@@ -453,21 +452,13 @@ def campaign_list(request):
             form_errors.append("Campaign name is required.")
         if audience not in Campaign.Audience.values:
             form_errors.append("Choose a recipient list.")
-        if audience == Campaign.Audience.POSTCARD_OPT_IN:
+        elif audience == Campaign.Audience.POSTCARD_OPT_IN:
             if not template or template.channel != Channel.POSTCARD:
                 form_errors.append(
                     "Choose a postcard design (create one under Postcard design)."
                 )
             if not body:
                 body = "Postcard mailing"
-        else:
-            if not body:
-                form_errors.append("Body is required.")
-            if (
-                audience == Campaign.Audience.POSTCARD_OPT_IN
-                and not subject
-            ):
-                form_errors.append("Subject is required for email campaigns.")
 
         scheduled_for = None
         try:
@@ -505,6 +496,7 @@ def campaign_list(request):
             "form_data": form_data,
             "form_errors": form_errors,
             "image_upload_url": reverse("directmail:campaign_image_upload"),
+            **utm_panel_context(live=True, medium=Channel.POSTCARD),
         },
     )
 
@@ -528,6 +520,7 @@ def campaign_detail(request, pk):
             "recent_events": ctx["recent_events"],
             "events_title": ctx["events_title"],
             "events_empty": ctx["events_empty"],
+            **utm_panel_context(campaign=campaign, live=False),
             "can_send": campaign.status
             in {
                 Campaign.Status.DRAFT,