Add the management app: club-facing UI + real fee-payment tracking

Gives clubs a self-service /manage/ area for members, families, teams,
roles, and season memberships, alongside real fee-payment tracking
(FeePayment, record_payment/mark_as_paid/remaining_balance) so a
membership's paid status reflects actual money received instead of a
single manually-set flag.

features/admin.py

@@ -0,0 +1,17 @@
+from django.contrib import admin
+
+from .models import Flag, Maintenance
+
+
+@admin.register(Flag)
+class FlagAdmin(admin.ModelAdmin):
+    list_display = ["name", "note", "everyone", "percent", "superusers", "staff"]
+    list_filter = ["everyone", "superusers", "staff"]
+    search_fields = ["name", "note"]
+    filter_horizontal = ["clubs"]
+
+
+@admin.register(Maintenance)
+class MaintenanceAdmin(admin.ModelAdmin):
+    list_display = ["__str__", "started_at", "started_by"]
+    readonly_fields = ["started_at", "started_by"]