feat(tenancy): resolve active club from request subdomain

Add row-based multi-tenancy plumbing keyed on Club as the tenant root:

- ClubTenantMiddleware maps the request's subdomain to a Club by slug,
  storing it on request.club and in a contextvar so service-layer code
  and management commands can read it via get_current_club(). Resolution
  honours CLUBMANAGER_BASE_DOMAIN (e.g. ajax-united.clubmanager.app),
  falling back to generic slug.example.com hosts, and ignores the bare
  base domain, www, and unknown slugs.
- Club gains a unique slug (auto-derived from name on save) plus a
  ClubManager.current() accessor for the active tenant.
- ClubScopedModel gets a TenantQuerySet (.for_club()/.current()) and
  auto-fills club from the active context on save.

Contextvar helpers live in club.tenancy; Club is imported lazily there
and in clubmanager.base to avoid an import cycle with club.models.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

clubmanager/settings.py

@@ -55,10 +55,16 @@ MIDDLEWARE = [
     "django.middleware.common.CommonMiddleware",
     "django.middleware.csrf.CsrfViewMiddleware",
     "django.contrib.auth.middleware.AuthenticationMiddleware",
+    "club.tenancy.ClubTenantMiddleware",
     "django.contrib.messages.middleware.MessageMiddleware",
     "django.middleware.clickjacking.XFrameOptionsMiddleware",
 ]
 
+# Multi-tenancy: base domain whose subdomains resolve to a club, e.g.
+# "ajax-united.clubmanager.app" -> the club with slug "ajax-united". Leave
+# unset to fall back to generic "slug.example.com" (3+ label) resolution.
+CLUBMANAGER_BASE_DOMAIN = config("CLUBMANAGER_BASE_DOMAIN", default="")
+
 ROOT_URLCONF = "clubmanager.urls"
 
 AUTH_USER_MODEL = "authentication.User"