Add campaign UTM links and piha.li shortener as a core helper.

Keep mint/UTM in always-on core so email_sms and directmail stay optional, and brand utm_source from SITE_NAME (or UTM_SOURCE) instead of a hardcoded client name.
Closes #3

site/core/views.py

@@ -1,9 +1,12 @@
 import io
+import json
 
+from django.contrib.auth.decorators import login_required
 from django.http import FileResponse, JsonResponse
 from django.shortcuts import get_object_or_404
-from django.views.decorators.http import require_GET
+from django.views.decorators.http import require_GET, require_POST
 
+from core.campaign_utm import resolve_campaign_tracked_url
 from core.models import StoredFile
 
 
@@ -24,3 +27,40 @@ def stored_file(request, pk):
         response["Content-Disposition"] = f'inline; filename="{stored.filename}"'
     response["Cache-Control"] = "public, max-age=86400"
     return response
+
+
+@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.
+    Does not import optional campaign apps — campaign_id is only an external_ref.
+    """
+    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() or 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,
+        }
+    )