Move calendar blueprint to app level for correct URL routing

The calendar blueprint was nested under calendars (admin), making URLs
/{slug}/admin/{calendar_slug}/ instead of /{slug}/{calendar_slug}/.

Register calendar blueprint directly on the app and update all endpoint
references from calendars.calendar.* to calendar.* (37 in Python,
~50 in templates).

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

events/bp/__init__.py

@@ -1,4 +1,5 @@
 from .all_events.routes import register as register_all_events
+from .calendar.routes import register as register_calendar
 from .calendars.routes import register as register_calendars
 from .markets.routes import register as register_markets
 from .payments.routes import register as register_payments