feat: per-page SumUp credentials in checkout flow (Phase 3)

- Add resolve_page_config() to determine PageConfig from cart/calendar context
- Set page_config_id on Order during checkout
- Pass page_config to SumUp create_checkout and build_sumup_reference
- check_sumup_status uses order.page_config for per-page credential resolution
- Fix: use session.flush() instead of g.s.flush() in check_sumup_status

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

bp/cart/services/check_sumup_status.py

@@ -1,10 +1,12 @@
 from suma_browser.app.payments.sumup import get_checkout as sumup_get_checkout
 from sqlalchemy import update
-from models.calendars import CalendarEntry  # NEW
+from models.calendars import CalendarEntry
 
 
 async def check_sumup_status(session, order):
-    checkout_data = await sumup_get_checkout(order.sumup_checkout_id)
+    # Use order's page_config for per-page SumUp credentials
+    page_config = getattr(order, "page_config", None)
+    checkout_data = await sumup_get_checkout(order.sumup_checkout_id, page_config=page_config)
     order.sumup_status = checkout_data.get("status") or order.sumup_status
     sumup_status = (order.sumup_status or "").upper()
 
@@ -26,10 +28,9 @@ async def check_sumup_status(session, order):
                 .where(*filters)
                 .values(state="provisional")
             )
-            # also clear cart for this user/session if it wasn't already
     elif sumup_status == "FAILED":
         order.status = "failed"
     else:
         order.status = sumup_status.lower() or order.status
 
-    await g.s.flush()
+    await session.flush()