Notify a member once when a roster/group change adds them to new events

Joining a team or group already re-synced Attendance rows for its upcoming
events (events/signals.py), but did so silently. A fresh membership (not a
field edit, not a removal) now sends one summary notification -- "N new
events on your calendar" -- rather than one per event, so joining mid-season
with a whole recurring series already scheduled doesn't flood the member
with pushes.

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

events/services/__init__.py

@@ -1,5 +1,6 @@
 from .attendance import (
     effective_members,
+    notify_newly_invited,
     player_attendance_rankings,
     players_who_missed_recent_practices,
     record_check_in,
@@ -24,6 +25,7 @@ __all__ = [
     "effective_members",
     "generate_occurrences",
     "horizon",
+    "notify_newly_invited",
     "occurrence_datetimes",
     "player_attendance_rankings",
     "players_who_missed_recent_practices",