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>

features/signals.py

@@ -0,0 +1,26 @@
+"""Keep waffle's flag cache honest when club targeting changes.
+
+waffle caches a flag's M2M ids and only flushes on ``save()``. Editing an M2M
+does not call ``save()``, so adding or removing a club would otherwise leave a
+stale cached set and the flag would keep answering with the old value.
+"""
+
+from django.db.models.signals import m2m_changed
+from django.dispatch import receiver
+
+from .models import Flag
+
+FLUSH_ACTIONS = {"post_add", "post_remove", "post_clear"}
+
+
+@receiver(m2m_changed, sender=Flag.clubs.through)
+def flush_flag_club_cache(sender, instance, action, reverse, pk_set, **kwargs):
+    if action not in FLUSH_ACTIONS:
+        return
+
+    if isinstance(instance, Flag):
+        instance.flush()
+    else:
+        # Reverse edit (club.flags.add(flag)): flush each flag touched.
+        for flag in Flag.objects.filter(pk__in=pk_set or []):
+            flag.flush()