feat: flapping detection suppresses notification storms

A (host, service) pair is flapping once it exceeds flap_count warning or
critical notifications within flap_interval minutes. The notification that
trips the state carries "Now flapping!! No more messages!" and every later
one is dropped, including RECOVER. The state ends silently flap_interval
minutes after a RECOVER, provided no further alert arrived meanwhile.

Hooked into notify.send_notification, the single choke point for channel
delivery, so only outbound notifications are suppressed — eventlog keeps
recording, leaving the journal and /log with the full history of the flap.

Threshold alerts key on their metric path, so a flapping disk check cannot
silence a CPU alert; connectivity, boot and shutdown events key on the host
itself. State lives at module level in flap.py and is never pickled.

Flapping pairs surface in Host.stateinfo() and render as an amber badge on
the live dashboard. Config: flap_count (5), flap_interval (10 minutes),
0 in either disables; both editable on the settings page.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

hbd/server/notify.py

@@ -25,6 +25,7 @@ from dataclasses import dataclass, field
 from typing import Optional
 
 from . import data
+from . import flap as flap_mod
 from . import ws as ws_mod
 
 logger = logging.getLogger(__name__)
@@ -63,6 +64,7 @@ class Notification:
     body: str           # detail message
     level: str          # RECOVER | WARNING | CRITICAL | INFO
     url: str = ""       # link to plugin metrics page
+    service: str = ""   # flap-detection key within the host ("" = the host itself)
 
 
 # ---------------------------------------------------------------------------
@@ -73,6 +75,7 @@ def setup(cfg: dict, loop: Optional[asyncio.AbstractEventLoop] = None):
     """Initialize notifier from configuration dict."""
     global _config, _loop
     _config = dict(cfg)
+    flap_mod.setup(_config)
     if loop is not None:
         _loop = loop
 
@@ -81,6 +84,7 @@ def reload_config(cfg: dict):
     """Reload notification configuration on SIGHUP."""
     global _config
     _config = dict(cfg)
+    flap_mod.setup(_config)
     logger.info("Notification configuration reloaded")
 
 
@@ -439,11 +443,22 @@ async def send_notification(host_name: str, notif: Notification) -> dict:
     notification_channels, and dispatches.  Silently does nothing if
     no users are configured.
 
+    Flap detection runs first: once *host_name*/*notif.service* is flapping the
+    notification is dropped, and the one that trips the state carries
+    ``flap.FLAP_MARKER``.
+
     Returns a dict of {channel_name: bool} results.
     """
     from . import users as users_mod
     from . import hbdclass
 
+    action = flap_mod.observe(host_name, notif.service, notif.level)
+    if action == flap_mod.SUPPRESS:
+        logger.debug("flapping: suppressed %s notification for %s", notif.level, host_name)
+        return {}
+    if action == flap_mod.TRIP:
+        notif.body = f"{notif.body} {flap_mod.FLAP_MARKER}"
+
     if not users_mod.users_enabled():
         return {}