chore: return formatted transcript url

- Return formatted transcript and recording URL
- Harden campaign dispatcher logic

api/utils/artifacts.py

@@ -0,0 +1,11 @@
+"""Helpers for workflow run artifact access."""
+
+from api.constants import BACKEND_API_ENDPOINT
+
+
+def artifact_url(
+    token: str | None, artifact: str, fallback: str | None = None
+) -> str | None:
+    if not token:
+        return fallback
+    return f"{BACKEND_API_ENDPOINT}/api/v1/public/download/workflow/{token}/{artifact}"