fix: clear flap state when a host is dropped

flap._state persists at module level keyed by (host, service) and only
clears via a RECOVER-triggered quiet window. A host dropped mid-flap with
no RECOVER ever received leaves ok_since permanently None, so the
flapping flag could never clear on its own.

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

tests/test_flap.py

@@ -98,6 +98,26 @@ def test_alert_during_the_quiet_window_keeps_it_flapping():
     assert flap.flapping_services("h1") == ["cpu"]
 
 
+# --- host removal ------------------------------------------------------------
+
+def test_clear_host_drops_flapping_state_even_without_a_recover():
+    # A host dropped mid-flap (no RECOVER ever received) must not stay
+    # flapping forever: nothing will ever set ok_since for it again.
+    alerts(4)
+    assert flap.flapping_services("h1") == ["cpu"]
+    flap.clear_host("h1")
+    assert flap.flapping_services("h1") == []
+    assert flap.observe("h1", "cpu", "CRITICAL") == flap.PASS
+
+
+def test_clear_host_only_affects_the_named_host():
+    alerts(4, host="h1")
+    alerts(4, host="h2")
+    flap.clear_host("h1")
+    assert flap.flapping_services("h1") == []
+    assert flap.flapping_services("h2") == ["cpu"]
+
+
 # --- keying -----------------------------------------------------------------
 
 def test_services_and_hosts_are_tracked_independently():