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/client_site/settings/base.py

@@ -2,6 +2,7 @@
 
 import json
 import os
+import sys
 from pathlib import Path
 from urllib.parse import urlparse
 
@@ -364,6 +365,20 @@ NOMINATIM_USER_AGENT = env(
 )
 NOMINATIM_API_KEY = env("NOMINATIM_API_KEY", "")
 
+# --- URL shortener (piha.li) ---
+# Call the API host, never the public short domain. Empty = skip minting
+# (composer falls back to the long UTM URL). Token format: name:secret.
+# Required in beta/prod when FEATURE_EMAIL_SMS or FEATURE_DIRECT_MAIL is on.
+SHORTENER_BASE_URL = env("SHORTENER_BASE_URL", "")
+SHORTENER_API_TOKEN = env("SHORTENER_API_TOKEN", "")
+SHORTENER_TIMEOUT_SECONDS = int(env("SHORTENER_TIMEOUT_SECONDS", "10") or "10")
+# Optional override for utm_source. Empty → slug of SITE_NAME.
+UTM_SOURCE = env("UTM_SOURCE", "")
+# manage.py test must not mint against a live API (env often has prod tokens).
+if "test" in sys.argv:
+    SHORTENER_BASE_URL = ""
+    SHORTENER_API_TOKEN = ""
+
 # --- Tianji analytics (pageviews + events; notice banner, not opt-in) ---
 TIANJI_ENABLED = env_bool("TIANJI_ENABLED", not DEBUG)
 TIANJI_TRACKER_URL = env(