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>

hbd/server/flap.py

@@ -100,6 +100,17 @@ def observe(host: str, service: str, level: str) -> str:
     return SUPPRESS
 
 
+def clear_host(host: str) -> None:
+    """Discard all flap state for *host* (called when a host is dropped).
+
+    A dropped host may be mid-flap with no RECOVER ever received, in which
+    case ``ok_since`` stays ``None`` and ``_sweep`` can never clear it on its
+    own — the state would otherwise persist forever.
+    """
+    for key in [k for k in _state if k[0] == host]:
+        del _state[key]
+
+
 def flapping_services(host: str) -> list:
     """Return the services of *host* that are currently flapping.