Mint campaign UTM links through the piha.li shortener (#10)

## Summary
Closes #9.

- Campaign composer auto-inserts a tracked homepage link (`utm_source=monica`, `utm_medium` = channel, `utm_campaign` = slug of the name) for email, SMS, and postcard QR — no manual UTM paste.
- When `SHORTENER_BASE_URL` + `SHORTENER_API_TOKEN` are set, the app mints that long HTTPS URL via `POST /api/links/` on the shortener **API host** and puts the returned `piha.li` / `beta.piha.li` short URL in SMS, email hrefs, and QR codes. Empty env (local) falls back to the long UTM URL.
- Live composer resolves shorts through a portal JSON endpoint (login + CSRF). Browser never calls the shortener.

## Secrets (control node, not git)

**monica_site** (`~/Documents/secrets/monica_site/`):
```
# prod
SHORTENER_BASE_URL=https://shortener.aimloperations.com
SHORTENER_API_TOKEN=monica:<secret>

# beta
SHORTENER_BASE_URL=https://shortener-beta.aimloperations.com
SHORTENER_API_TOKEN=monica:<beta-secret>
```

**url_shortening_service** (same secret, named token):
```
SHORTENER_API_TOKENS=monica:<secret>
SHORT_ALLOWED_HOSTS=mkdrealtor.com,aimloperations.com
```
Prod public short host: `piha.li`. Beta: `beta.piha.li`. Generate with `python -c "import secrets; print(secrets.token_urlsafe(32))"`.

Template port: westfarn/web_django_template#3

## Test plan
- [ ] `cd site && uv run python manage.py test messaging.tests.CampaignUtmLinkTests`
- [ ] Composer: type a campaign name — email gets an HTML link, SMS gets a URL, postcard shows a QR
- [ ] With shortener env set: SMS/QR show `piha.li` (or `beta.piha.li`); without it, long UTM URL still works
- [ ] Copy/download QR into postcard designer
- [ ] Secret files have `SHORTENER_*` on both caller and operator sides before beta/prod deploy

Reviewed-on: #10

site/messaging/views.py

@@ -1,6 +1,7 @@
 import hashlib
 import hmac
 import io
+import json
 import logging
 
 from django.conf import settings
@@ -26,12 +27,17 @@ from messaging.providers.postcard.pcm import (
     list_designs,
 )
 from messaging.services import (
+    channel_for_audience,
     create_campaign_draft,
     enqueue_campaign_send,
+    ensure_campaign_utm_link,
     message_is_removable,
     opted_in_contacts,
     parse_scheduled_for,
+    public_site_base_url,
+    public_site_link_label,
     record_sms_stop,
+    resolve_campaign_tracked_url,
     send_campaign_test_email,
 )
 from messaging.webhooks import (
@@ -462,6 +468,16 @@ def campaign_list(request):
             if not body:
                 body = "Postcard mailing"
         else:
+            if audience in Campaign.Audience.values:
+                channel = channel_for_audience(audience)
+                if channel in (Channel.EMAIL, Channel.SMS):
+                    body = ensure_campaign_utm_link(
+                        body,
+                        name=name or "campaign",
+                        medium=channel,
+                        html=(channel == Channel.EMAIL),
+                    )
+                    form_data["body"] = body
             if not body:
                 form_errors.append("Body is required.")
             if (
@@ -506,10 +522,62 @@ def campaign_list(request):
             "form_data": form_data,
             "form_errors": form_errors,
             "image_upload_url": reverse("messaging:campaign_image_upload"),
+            "utm_base_url": public_site_base_url(),
+            "utm_link_label": public_site_link_label(),
+            "utm_shorten_url": reverse("messaging:campaign_short_link"),
+            "utm_url": "",
         },
     )
 
 
+@login_required
+@require_POST
+def campaign_short_link(request):
+    """Mint (or reuse) a short URL for the campaign tracked landing link.
+
+    Browser talks to this portal endpoint only. The shortener is server-to-server.
+    """
+    try:
+        payload = json.loads(request.body.decode() or "{}")
+    except (json.JSONDecodeError, UnicodeDecodeError):
+        payload = {}
+    if not isinstance(payload, dict):
+        payload = {}
+    name = (payload.get("name") or request.POST.get("name") or "").strip()
+    medium = (payload.get("medium") or request.POST.get("medium") or "").strip()
+    campaign_id = (
+        payload.get("campaign_id") or request.POST.get("campaign_id") or ""
+    )
+    campaign_id = str(campaign_id).strip()
+    if campaign_id:
+        try:
+            campaign = Campaign.objects.filter(pk=campaign_id).first()
+        except (ValidationError, ValueError):
+            campaign = None
+        if campaign is None:
+            return JsonResponse({"detail": "Campaign not found."}, status=404)
+        name = name or campaign.name
+        medium = medium or campaign.channel
+        campaign_id = str(campaign.pk)
+    else:
+        campaign_id = None
+    target, display = resolve_campaign_tracked_url(
+        name=name or "campaign",
+        medium=medium,
+        campaign_id=campaign_id,
+        user_id=getattr(request.user, "pk", None),
+        shorten=True,
+    )
+    return JsonResponse(
+        {
+            "target_url": target,
+            "short_url": display if display != target else "",
+            "display_url": display,
+            "shortened": display != target,
+        }
+    )
+
+
 @login_required
 def campaign_detail(request, pk):
     campaign = get_object_or_404(Campaign, pk=pk)
@@ -518,6 +586,12 @@ def campaign_detail(request, pk):
     except (TypeError, ValueError):
         page = 1
     ctx = _campaign_report(campaign, page=page)
+    _target, display_url = resolve_campaign_tracked_url(
+        name=campaign.name,
+        medium=campaign.channel,
+        campaign_id=campaign.pk,
+        shorten=True,
+    )
     return render(
         request,
         "messaging/campaign_detail.html",
@@ -529,6 +603,10 @@ def campaign_detail(request, pk):
             "recent_events": ctx["recent_events"],
             "events_title": ctx["events_title"],
             "events_empty": ctx["events_empty"],
+            "utm_url": display_url,
+            "utm_base_url": public_site_base_url(),
+            "utm_link_label": public_site_link_label(),
+            "utm_shorten_url": reverse("messaging:campaign_short_link"),
             "can_send": campaign.status
             in {
                 Campaign.Status.DRAFT,