Speed up and rationalise the test suite (158s -> 16s)

Nearly all of the wall clock was password hashing: there was no test-time
PASSWORD_HASHERS override, so Django's PBKDF2 default (~1.2M iterations) ran on
every create_user and every login, hundreds of times over. The fix lives in a
DiscoverRunner subclass wired in via TEST_RUNNER rather than a "test" in
sys.argv sniff in settings: a runner is only ever instantiated by `manage.py
test`, so there is no env var to mis-set and no import path by which a deployed
process can reach the weak hasher. Verified: outside the runner the hasher is
still PBKDF2. It also enables the cached template loader (the runner forces
DEBUG off *after* settings are read, so Django never turns it on by itself) and
silences django.request, whose 4xx/5xx logging buried real test output.

Second, the fixtures. Base classes were rebuilding a club, season, admin user,
membership, role and MFA authenticator once per test; those are read-only for
almost every test, so they move to setUpTestData and are built once per class.
Django hands each test its own deep copy and the per-test transaction rolls the
rows back, so the handful of tests that mutate them stay isolated -- proved with
--shuffle, --reverse and --parallel rather than assumed. Per-test work that
genuinely must stay per-test (client sign-ins, waffle cache clears that leak
across the transaction boundary) is left in setUp with a comment saying why.

Five tests removed, each strictly subsumed by another that asserts a superset;
their intent was folded into a comment on the survivor. Regression-pinning
tests -- the ones carrying comments naming the exact bug they catch -- were
left verbatim throughout.

Also closes a real gap this surfaced: teams had a cross-club position test for
TeamMembership but not for StaffAssignment, with an unused `other_coach`
fixture sitting there waiting for it.

Rejected: --parallel by default (every worker re-runs all 88 migrations, buying
~4s of wall clock for ~5x the CPU), and disabling migrations in tests (~3.5s,
but the schema would then come from models and the suite would stop catching a
broken migration).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

features/tests.py

@@ -17,15 +17,19 @@ User = get_user_model()
 
 
 class ClubScopedFlagTests(TestCase):
+    @classmethod
+    def setUpTestData(cls):
+        cls.club = Club.objects.create(name="Ajax United", slug="ajax-united")
+        cls.other = Club.objects.create(name="Rival FC", slug="rival-fc")
+        cls.flag = Flag.objects.create(name="shop")
+
     def setUp(self):
         # waffle caches flags by name, and its cache is NOT rolled back with the
-        # test transaction -- a flag row recreated under the same name in the next
-        # test would otherwise be shadowed by the previous test's cached object.
+        # test transaction -- a flag row whose targeting changed in the previous
+        # test would otherwise be shadowed by that test's cached object. Has to
+        # stay per-test: it is the cache, not the rows, that leaks.
         cache.clear()
         self.addCleanup(cache.clear)
-        self.club = Club.objects.create(name="Ajax United", slug="ajax-united")
-        self.other = Club.objects.create(name="Rival FC", slug="rival-fc")
-        self.flag = Flag.objects.create(name="shop")
 
     def request_for(self, club):
         request = RequestFactory().get("/")
@@ -118,12 +122,17 @@ class ClubScopedFlagTests(TestCase):
     ALLOWED_HOSTS=["rosterchief.app", "ajax-united.rosterchief.app", "testserver"],
 )
 class MaintenanceModeTests(TestCase):
+    @classmethod
+    def setUpTestData(cls):
+        cls.club = Club.objects.create(name="Ajax United", slug="ajax-united")
+        cls.user = User.objects.create_user(email="root@example.com", password="pw-secret-123", is_staff=True)
+        Authenticator.objects.create(user=cls.user, type=Authenticator.Type.TOTP, data={"secret": "JBSWY3DPEHPK3PXP"})
+
     def setUp(self):
+        # Maintenance state lives in the shared cache, which no transaction rolls
+        # back -- each test has to start from a reopened platform.
         cache.clear()
         self.addCleanup(cache.clear)
-        self.club = Club.objects.create(name="Ajax United", slug="ajax-united")
-        self.user = User.objects.create_user(email="root@example.com", password="pw-secret-123", is_staff=True)
-        Authenticator.objects.create(user=self.user, type=Authenticator.Type.TOTP, data={"secret": "JBSWY3DPEHPK3PXP"})
 
     def club_get(self, path="/"):
         return self.client.get(path, HTTP_HOST="ajax-united.rosterchief.app")