Add club-scoped feature flags on django-waffle

Adds a `features` app with a swappable waffle Flag (WAFFLE_FLAG_MODEL) that
gains a m2m to Club, so a feature can be rolled out club by club.

Two things worth calling out:

- `everyone` keeps waffle's contract of overriding *all* other targeting, so
  club targeting is only consulted when `everyone is None`. This keeps
  `everyone = False` usable as a hard kill-switch.
- m2m edits don't call save(), so waffle's per-flag cache would go stale when
  clubs are added or removed. A m2m_changed receiver flushes it from both
  directions, and get_flush_keys() drops the club-set key alongside waffle's own.

Note for future flag tests: waffle's cache is not rolled back with the test
transaction, so tests touching flags must clear it (see features/tests.py).

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

pyproject.toml

@@ -8,6 +8,7 @@ dependencies = [
     "django-allauth[mfa]>=65.18.0",
     "django-lucide",
     "django-phonenumber-field[phonenumbers]>=8.4.0",
+    "django-waffle>=5.0.0",
     "pillow>=12.3.0",
     "python-dateutil>=2.9.0.post0",
     "python-decouple>=3.8",