Fix housekeeper false-positive overdue errors and ended-member birthday/anniversary noise

An overdue claim whose reminder had already been sent, with that reminder's own
payment deadline still running, fell through to the plain "überfällig" error
finding whenever reminder_due's escalation check returned None to signal "not
yet time for the next Mahnstufe" -- that None was indistinguishable from "never
handled at all" to the caller, so it looked neglected even though a reminder was
already out. Give that case its own low-priority "reminder_awaiting_deadline"
finding instead of silently falling back to the overdue-error path.

Birthday and membership-anniversary findings also kept firing for members whose
status is "ended", which doesn't make sense once the membership itself is over.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/ccma/rules/scripts/membership_anniversary.py

@@ -8,6 +8,8 @@ ORDER = 30
 
 def evaluate(context: RuleContext):
     member = context.member
+    if member.status == "ended":
+        return []
     try:
         started_at = parse_iso_date(member.membership_started_at, "Mitglied seit")
     except DateValidationError: