feat: add markets and payments management pages

- New markets blueprint at /<slug>/markets/ with create/delete
- New payments blueprint at /<slug>/payments/ with SumUp config
- Register both in events app with context processor for markets
- Remove PageConfig feature flag check from calendar creation
  (feature toggles replaced by direct management pages)

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

bp/__init__.py

@@ -1 +1,3 @@
 from .calendars.routes import register as register_calendars
+from .markets.routes import register as register_markets
+from .payments.routes import register as register_payments