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

club/services/onboarding.py

@@ -172,6 +172,28 @@ def blocked_member_ids_for_event(club, season, event_kind) -> set:
     return blocked_member_ids
 
 
+def open_requirements_blocking(member, club, season, event_kind) -> list:
+    """The specific, still-open `OnboardingRequirement`s blocking `member` from
+    `event_kind` events this season -- the per-member, explain-*why* mirror of
+    `blocked_member_ids_for_event`'s bulk set. Built for the member-facing "you
+    can't sign up for this yet" card (mobile app): unlike that function, this
+    one is meant to be called once for a person looking at one event, not once
+    per event for a whole club, so the per-member cost here is fine.
+
+    Empty (never blocked) for a member with no current-season ClubMembership.MEMBER
+    row -- same "nothing to check" reasoning as blocked_member_ids_for_event's own."""
+    membership = ClubMembership.objects.filter(club=club, season=season, member=member, kind=ClubMembership.Kind.MEMBER).first()
+    if membership is None:
+        return []
+
+    blocking = [requirement for requirement in OnboardingRequirement.objects.filter(club=club, is_active=True) if event_kind in requirement.blocked_event_kinds]
+    if not blocking:
+        return []
+
+    resolved_ids = set(MemberRequirementStatus.objects.filter(membership=membership, requirement__in=blocking).filter(_RESOLVED).values_list("requirement_id", flat=True))
+    return [requirement for requirement in blocking if requirement.pk not in resolved_ids]
+
+
 #: Fee states "clean" enough to activate on -- PARTIALLY_PAID/UNPAID never are.
 _CLEAN_FEE_STATUSES = (ClubMembership.FeeStatus.PAID, ClubMembership.FeeStatus.WAIVED)