Add suff drink booking tool with PIN auth

Self-service drink tab at /suff/ for festival attendees. Users log in
with username + 3-digit PIN stored in a separate User.pin field, so
staff/admin accounts can keep their strong password for /admin/ and
also use the drink tool with the same username. PINs for staff users
must be set from the admin panel via a dedicated "PIN setzen" view to
prevent account takeover by name collision.

Time-gated to the festival window (Thu–Sun in Berlin tz) with phases
before/booking/readonly/closed; in non-production mode the tool is
always in booking phase for local testing.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

gaehsnitzproject/settings.py

@@ -101,3 +101,10 @@ USE_TZ = True
 STATIC_URL = "/static/"
 
 AUTH_USER_MODEL = "gaehsnitz.User"
+
+AUTHENTICATION_BACKENDS = [
+    "django.contrib.auth.backends.ModelBackend",
+    "gaehsnitz.auth_backends.PinBackend",
+]
+
+LOGIN_URL = "/suff/"