Split club signups into new members and renewals

A "New members" card sits beside Renewal, and the signups chart becomes a stacked
bar: bar height stays "signups this month" while the split shows where they came
from.

New means "first-ever season at this club", not "signed up recently". A member who
lapsed for a year and came back is a renewal, and counting them as new would
flatter every recovery into growth. It is also per club, not per platform: someone
who plays for another club is still new here.

The split resolves each member's earliest season once up front rather than asking
per row, so the chart costs two queries instead of one per membership.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

controlpanel/services/statistics.py

@@ -5,6 +5,7 @@ means adding an entry here and nothing else. ``clubs_with_totals`` annotates in
 a single query — the club list must not fan out into N+1.
 """
 
+from collections import defaultdict
 from datetime import timedelta
 from decimal import Decimal
 
@@ -181,6 +182,49 @@ def renewal_rate(club, season):
     return round(100 * returned / total)
 
 
+def new_members(club, season):
+    """Members whose first-ever season at this club is ``season``.
+
+    Keyed on "has no membership in an earlier season", not on "signed up recently" — a
+    member who lapsed for a year and came back is a renewal, not a new member, and
+    counting them as new would flatter every recovery into growth.
+    """
+    if season is None:
+        return Member.objects.none()
+
+    seen_before = ClubMembership.objects.filter(club=club, season__start_date__lt=season.start_date).values("member")
+
+    return Member.objects.filter(member_of__club=club, member_of__season=season).exclude(pk__in=seen_before).distinct()
+
+
+def signup_split(club, months=MONTHS_OF_HISTORY):
+    """Signups per month, split into first-timers and returners.
+
+    Which season a signup belongs to decides the split, so the member's earliest season at
+    this club is looked up once for everyone rather than per row — the same question asked
+    inside a loop is a query per membership.
+    """
+    start = (timezone.now() - timedelta(days=30 * months)).replace(day=1, hour=0, minute=0, second=0, microsecond=0)
+
+    first_season = {}
+    for member_id, season_start in ClubMembership.objects.filter(club=club).values_list("member_id", "season__start_date"):
+        if member_id not in first_season or season_start < first_season[member_id]:
+            first_season[member_id] = season_start
+
+    counts = defaultdict(lambda: {"new": 0, "returning": 0})
+    for member_id, season_start, signed_up_at in ClubMembership.objects.filter(club=club, signed_up_at__isnull=False, signed_up_at__gte=start).values_list("member_id", "season__start_date", "signed_up_at"):
+        kind = "new" if season_start == first_season[member_id] else "returning"
+        counts[signed_up_at.strftime("%Y-%m")][kind] += 1
+
+    series, cursor = [], start
+    while cursor <= timezone.now():
+        month = counts[cursor.strftime("%Y-%m")]
+        series.append({"month": cursor.strftime("%b %Y"), "new": month["new"], "returning": month["returning"]})
+        cursor = (cursor + timedelta(days=32)).replace(day=1)
+
+    return series
+
+
 def teams_without_a_manager(club, season):
     """Teams with nobody in a management position this season.
 
@@ -258,6 +302,7 @@ def club_attention(club):
         "pending_approvals": memberships.filter(status=ClubMembership.StatusChoices.PENDING).count(),
         "teams_without_manager": teams_without_a_manager(club, season).count(),
         "unrostered": unrostered_members(club, season).count(),
+        "new_members": new_members(club, season).count(),
         "renewal_rate": renewal_rate(club, season),
         "attendance": attendance_rates(club, season),
     }
@@ -268,7 +313,7 @@ def club_charts(club):
     memberships = ClubMembership.objects.filter(club=club, season=season) if season else ClubMembership.objects.none()
 
     return {
-        "signups": _monthly(ClubMembership.objects.filter(club=club, signed_up_at__isnull=False), "signed_up_at", Count("id")),
+        "signups": signup_split(club),
         # Fee status this season, in the order a treasurer cares about.
         "fees": [
             {"label": label, "value": memberships.filter(fee_status=status).count()}