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/models.py

@@ -22,6 +22,31 @@ class Team(ClubScopedModel):
         return self.name
 
 
+def team_photo_path(instance, filename):
+    return f"clubs/{instance.team.club.slug}/teams/{instance.team_id}/{instance.season.name}/{filename}"
+
+
+class TeamPhoto(UUIDModel):
+    """One team photo per season -- a team's makeup changes every season, so
+    this can't be a plain field on Team. No club FK of its own: club is
+    already reachable via team.club, same reasoning as NewsPhoto being owned
+    by News rather than club-scoped itself."""
+
+    team = models.ForeignKey(Team, on_delete=models.CASCADE, related_name="photos", verbose_name=_("team"))
+    season = models.ForeignKey(Season, on_delete=models.PROTECT, related_name="team_photos", verbose_name=_("season"))
+    image = models.ImageField(_("image"), upload_to=team_photo_path)
+
+    class Meta:
+        verbose_name = _("team photo")
+        verbose_name_plural = _("team photos")
+        constraints = [
+            models.UniqueConstraint(fields=["team", "season"], name="unique_team_photo_per_season"),
+        ]
+
+    def __str__(self):
+        return f"{self.team} - {self.season}"
+
+
 class Position(ClubScopedModel):
     name = models.CharField(_("name"), max_length=255)
     short_name = models.CharField(_("short name"), max_length=255)