feat: suppress alerts for unwatched hosts

Hosts with watch: false in config no longer appear in the Alerts page
or nav bar alert counts. Events still appear in the Log of Events.
Hosts without a config entry default to watch: false.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

hbd/server/http.py

@@ -325,6 +325,8 @@ async def start(
         from .threshold import AlertLevel
         critical = warning = ok = 0
         for host in hbdclass.Host.hosts.values():
+            if not host.watched:
+                continue
             if not _can_operate_host(user, host):
                 continue
             levels = {s.level for s in host.alert_states.values()}
@@ -595,6 +597,8 @@ async def start(
         all_alerts = []
 
         for hostname, host in hbdclass.Host.hosts.items():
+            if not host.watched:
+                continue
             if not _can_view_host(user, host):
                 continue
             if threshold_checker: