Move payments admin from events to cart service

Payments config (SumUp credentials per page) is a cart concern since all
checkouts go through the cart service. Moves it from events.rose-ash.com
to cart.rose-ash.com/<page_slug>/admin/payments/ and adds a cart admin
overview page at /<page_slug>/admin/.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

events/sexp/header.sexpr

@@ -21,9 +21,6 @@
 (defcomp ~events-markets-label ()
   (<> (i :class "fa fa-shopping-bag" :aria-hidden "true") (div "Markets")))
 
-(defcomp ~events-payments-label ()
-  (<> (i :class "fa fa-credit-card" :aria-hidden "true") (div "Payments")))
-
 (defcomp ~events-calendar-label (&key name description)
   (div :class "flex flex-col md:flex-row md:gap-2 items-center min-w-0"
     (div :class "flex flex-row items-center gap-2"