Show blocked sign-ups on mobile instead of silently hiding the event

A member excluded from an event's Attendance sync by an open onboarding
requirement (events.services.attendance.effective_members) previously just
never saw that event anywhere -- no row, no explanation. Two new read-side
functions mirror that exclusion instead of hiding it: club.services.
onboarding.open_requirements_blocking (per-member, "why") and events.
services.attendance.blocked_upcoming_events_for_member (which of their
upcoming events are affected).

The Calendar now shows those events as a distinct muted "Blocked" row
naming the outstanding requirement, and the event detail page shows a
"Can't sign up yet" card for the same reason -- no RSVP buttons, no lineup/
referee actions, just the explanation. Write-side blocking (who actually
gets an Attendance row, who a coach can select) is untouched.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ECGMEwrc2k4D8VQuwjstj9

events/services/__init__.py

@@ -1,4 +1,5 @@
 from .attendance import (
+    blocked_upcoming_events_for_member,
     effective_members,
     notify_newly_invited,
     player_attendance_rankings,
@@ -20,6 +21,7 @@ from .recurrence import (
 
 __all__ = [
     "apply_template",
+    "blocked_upcoming_events_for_member",
     "cancel_occurrence",
     "detach_occurrence",
     "effective_members",