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/http.py

@@ -14,6 +14,7 @@ import logging
 from aiohttp import web
 import jinja2
 from . import data
+from . import flap as flap_mod
 from . import notify as notify_mod
 from . import settings as settings_mod
 from . import users as users_mod
@@ -439,6 +440,7 @@ async def start(
             return web.json_response({"error": "Forbidden"}, status=403)
         eventlog(uname, "INFO", "dropped")
         del hbdclass.Host.hosts[uname]
+        flap_mod.clear_host(uname)
         return web.Response(text="Done")
 
     async def register(request):