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

club/admin.py

@@ -1,17 +1,25 @@
 from django.contrib import admin
 from django.utils.translation import gettext_lazy as _
 
-from .models import Club, ClubMembership, ClubRole, FeePayment, Season
+from .models import Club, ClubMembership, ClubRole, FeePayment, Season, Sponsor
 
 
 @admin.register(Club)
 class ClubAdmin(admin.ModelAdmin):
-    list_display = ["name", "slug"]
+    list_display = ["name", "slug", "sport_type"]
+    list_filter = ["sport_type"]
     search_fields = ["name", "slug"]
     prepopulated_fields = {"slug": ["name"]}
     ordering = ["name"]
 
 
+@admin.register(Sponsor)
+class SponsorAdmin(admin.ModelAdmin):
+    list_display = ["name", "club", "start_date", "end_date"]
+    list_filter = ["club"]
+    search_fields = ["name"]
+
+
 @admin.register(Season)
 class SeasonAdmin(admin.ModelAdmin):
     list_display = ["__str__", "club", "start_date", "end_date"]