Add a dues-invoicing feature: send, track and remind on membership fees

New DuesInvoice model (one per membership, resendable) plus
club.services.invoicing: resolves the best email to invoice (the
member's own, else a parent/guardian's), snapshots the outstanding
balance and a due date on send, and mails a branded HTML invoice
(same club-colour email shell as the parent-claim email) with a
WeasyPrint PDF attached when the native libs are available.

Dues & billing gains a bulk "Send invoice" action (checkbox selection
+ a shared due-in-days prompt), a per-row invoice status column, a
staff-facing invoice detail/PDF page, and a push-button "Send
reminders" action for every sent, unpaid invoice past its own due
date.

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

club/admin.py

@@ -1,7 +1,7 @@
 from django.contrib import admin
 from django.utils.translation import gettext_lazy as _
 
-from .models import Club, ClubMembership, ClubRole, FeePayment, MemberRequirementStatus, OnboardingRequirement, Season, Sponsor
+from .models import Club, ClubMembership, ClubRole, DuesInvoice, FeePayment, MemberRequirementStatus, OnboardingRequirement, Season, Sponsor
 
 
 @admin.register(Club)
@@ -57,6 +57,15 @@ class FeePaymentAdmin(admin.ModelAdmin):
     search_fields = ["membership__club__name", "membership__member__last_name", "reference"]
 
 
+@admin.register(DuesInvoice)
+class DuesInvoiceAdmin(admin.ModelAdmin):
+    list_display = ["number", "membership", "amount", "due_date", "sent_at", "reminder_count"]
+    list_filter = ["club"]
+    search_fields = ["number", "membership__member__last_name", "membership__member__first_name"]
+    raw_id_fields = ["membership"]
+    readonly_fields = ["number"]
+
+
 @admin.register(ClubRole)
 class ClubRoleAdmin(admin.ModelAdmin):
     list_display = ["club__name", "member__last_name", "member__first_name", "role"]