Add billing-ending banner, events CRUD, RBIHF import, public API, team photos, and sponsors

A large batch of club-management features built up over one session:

- Club dashboard banner warning admins 1 month before billing ends
- Full Events/EventSeries CRUD (recurrence builder, occurrence lifecycle,
  per-team permissions), with match->game rename and game-specific fields
  (score, competition, live status, external game ID)
- Django-admin competition dropdown, gated per-club by feature flag
- Auto-import of RBIHF fixtures (scrape -> diff -> preview -> confirm),
  with location/opponent dropdowns suggested from existing club data
- Feature-flag-gated Shop/Forms nav sections, reusing the same flag
  machinery for the RBIHF import button
- Team roster now scoped to members active this season or next, sorted and
  grouped by position
- Club sport type (ice hockey / other), shown in the control panel's club
  subtitle
- Per-season team photo upload from the team page
- New public read-only API (Django Ninja) at /api/v1/: news, team rosters,
  upcoming/live/per-team games, and sponsors -- auto-documented via Swagger
  UI, CORS-enabled for a club's own external website
- Club sponsors: admin-only CRUD (logo, URL, active date window) plus a
  date-windowed, optionally randomized API endpoint
- Assorted fixes: NullBooleanField dropdown rendering, cross-club event
  validation timing, searchable-select chip placement, btn-neutral ->
  default button style sweep, calendar-month chart windows

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01R1gj3J1QPfP38XWpnpbFpy

teams/admin.py

@@ -1,7 +1,7 @@
 from django.contrib import admin
 from django.utils.translation import gettext_lazy as _
 
-from .models import Position, StaffAssignment, Team, TeamMembership
+from .models import Position, StaffAssignment, Team, TeamMembership, TeamPhoto
 
 
 class TeamMembershipInline(admin.TabularInline):
@@ -20,12 +20,26 @@ class StaffAssignmentInline(admin.TabularInline):
     raw_id_fields = ("member",)
 
 
+class TeamPhotoInline(admin.TabularInline):
+    """One photo per season, shown on the Team page."""
+
+    model = TeamPhoto
+    extra = 0
+
+
 @admin.register(Team)
 class TeamAdmin(admin.ModelAdmin):
     list_display = ["name", "short_name", "club"]
     list_filter = ["club"]
     search_fields = ["name", "short_name"]
-    inlines = [TeamMembershipInline, StaffAssignmentInline]
+    inlines = [TeamMembershipInline, StaffAssignmentInline, TeamPhotoInline]
+
+
+@admin.register(TeamPhoto)
+class TeamPhotoAdmin(admin.ModelAdmin):
+    list_display = ["team", "season"]
+    list_filter = ["team__club", "season"]
+    search_fields = ["team__name"]
 
 
 @admin.register(Position)