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/templates/alerts.html

@@ -94,6 +94,24 @@
       border-color: #2196f3;
     }
 
+    .filter-input {
+      padding: 7px 12px;
+      border: 2px solid #ddd;
+      border-radius: 20px;
+      font-size: 0.9em;
+      outline: none;
+      width: 200px;
+      transition: border-color 0.2s;
+    }
+
+    .filter-input:focus {
+      border-color: #2196f3;
+    }
+
+    .filter-input.invalid {
+      border-color: #f44336;
+    }
+
     .alerts-container {
       background: white;
       border-radius: 8px;
@@ -316,6 +334,7 @@
         <button class="filter-button active" onclick="filterAlerts('all')">All</button>
         <button class="filter-button" onclick="filterAlerts('critical')">Critical Only</button>
         <button class="filter-button" onclick="filterAlerts('warning')">Warning Only</button>
+        <input id="host-filter" class="filter-input" type="text" placeholder="host filter (regex)" oninput="onHostFilterInput(this)">
       </div>
 
       <div class="alerts-container">
@@ -332,6 +351,7 @@
     <script>
       let currentFilter = 'all';
       let allAlerts = [];
+      let hostFilterRe = null;
 
       async function loadAlerts() {
         try {
@@ -366,10 +386,13 @@
         // Filter alerts based on current filter
         let filteredAlerts = alerts;
         if (currentFilter !== 'all') {
-          filteredAlerts = alerts.filter(alert => 
+          filteredAlerts = filteredAlerts.filter(alert =>
             alert.level.toLowerCase() === currentFilter
           );
         }
+        if (hostFilterRe) {
+          filteredAlerts = filteredAlerts.filter(alert => hostFilterRe.test(alert.hostname));
+        }
         
         if (filteredAlerts.length === 0) {
           if (currentFilter === 'all' && alerts.length === 0) {
@@ -538,9 +561,36 @@
         }
       }
 
+      function onHostFilterInput(input) {
+        const val = input.value.trim();
+        if (!val) {
+          hostFilterRe = null;
+          input.classList.remove('invalid');
+        } else {
+          try {
+            hostFilterRe = new RegExp(val, 'i');
+            input.classList.remove('invalid');
+          } catch (_) {
+            hostFilterRe = null;
+            input.classList.add('invalid');
+          }
+        }
+        renderAlerts(allAlerts);
+      }
+
       // Auto-refresh every 15 seconds
       setInterval(loadAlerts, 15000);
 
+      // Initialise filter from URL query string (?filter=...)
+      (function () {
+        const param = new URLSearchParams(window.location.search).get('filter');
+        if (param) {
+          const input = document.getElementById('host-filter');
+          input.value = param;
+          onHostFilterInput(input);
+        }
+      })();
+
       // Initial load
       loadAlerts();
     </script>