feat: alerts host-filter field with URL query param and notify URL

- Add regex filter input to the Alerts dashboard that filters displayed
  hosts on every keystroke; invalid regex turns the border red
- Initialise the filter from ?filter= in the URL query string
- Change _build_url() to produce /alerts?filter=<hostname> so
  notification links (Pushover, email, Matrix, etc.) land on the
  alerts page pre-filtered to the alerting host

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

hbd/server/notify.py

@@ -401,7 +401,7 @@ def _build_url(host_name: str) -> str:
     base_url = _config.get("base_url", "").rstrip("/")
     if not base_url:
         return ""
-    return f"{base_url}/plugins#{host_name}"
+    return f"{base_url}/alerts?filter={host_name}"
 
 
 async def send_notification(host_name: str, notif: Notification) -> dict: