fix(security): reject WebAuthn signature-counter rollback

Sign counters were stored but never checked, so a cloned authenticator or a
replayed assertion with an equal/lower counter was accepted. Reject the
authentication when the presented counter does not exceed the stored one,
while allowing counter-less/synced passkeys that always report 0.

Refs: porchlight-3cr

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_authn/test_sign_count.py

@@ -0,0 +1,22 @@
+from porchlight.authn.routes import _is_sign_count_rollback
+
+
+def test_increasing_counter_is_not_rollback() -> None:
+    assert _is_sign_count_rollback(stored=5, presented=6) is False
+
+
+def test_equal_counter_is_rollback() -> None:
+    assert _is_sign_count_rollback(stored=5, presented=5) is True
+
+
+def test_lower_counter_is_rollback() -> None:
+    assert _is_sign_count_rollback(stored=5, presented=3) is True
+
+
+def test_both_zero_sync_passkey_is_allowed() -> None:
+    # Sync passkeys (and counter-less authenticators) always report 0.
+    assert _is_sign_count_rollback(stored=0, presented=0) is False
+
+
+def test_first_increment_from_zero_is_allowed() -> None:
+    assert _is_sign_count_rollback(stored=0, presented=1) is False