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>

cart/app.py

@@ -16,6 +16,7 @@ from bp import (
     register_cart_overview,
     register_page_cart,
     register_cart_global,
+    register_page_admin,
     register_fragments,
     register_actions,
     register_data,
@@ -195,6 +196,12 @@ def create_app() -> "Quart":
         url_prefix="/",
     )
 
+    # Page admin at /<page_slug>/admin/ (before page_cart catch-all)
+    app.register_blueprint(
+        register_page_admin(),
+        url_prefix="/<page_slug>/admin",
+    )
+
     # Page cart at /<page_slug>/ (dynamic, matched last)
     app.register_blueprint(
         register_page_cart(url_prefix="/"),