Add heartbeat monitoring, alert rules, cert badges, webhook templates, and status page visibility

- Heartbeat type: cron jobs POST to /api/heartbeat/:token to stay alive; scheduler marks
  them down if no ping is received within the configured timeout window
- Alert rules: per-resource consecutive_failures_threshold now respected in handleIncident,
  overriding the global setting; GET /api/resources/:id/alert-rules endpoint added
- SSL cert expiry badge: dashboard rows surface days_remaining for TLS monitors, turning
  orange at ≤30 days and red at ≤7 days
- Webhook payload templates: {{name}}/{{status}}/{{url}}/{{type}}/{{message}}/{{timestamp}}
  variables interpolated before POST; Slack-compatible text wrapping as fallback
- Status page visibility: is_public column per resource; GET /api/status-page returns only
  public resources; Status.js updated to use the new endpoint

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

client/build/asset-manifest.json

@@ -1,15 +1,15 @@
 {
   "files": {
     "main.css": "/static/css/main.6034edfe.css",
-    "main.js": "/static/js/main.243399c3.js",
+    "main.js": "/static/js/main.16dc004f.js",
     "static/js/453.d456ce70.chunk.js": "/static/js/453.d456ce70.chunk.js",
     "index.html": "/index.html",
     "main.6034edfe.css.map": "/static/css/main.6034edfe.css.map",
-    "main.243399c3.js.map": "/static/js/main.243399c3.js.map",
+    "main.16dc004f.js.map": "/static/js/main.16dc004f.js.map",
     "453.d456ce70.chunk.js.map": "/static/js/453.d456ce70.chunk.js.map"
   },
   "entrypoints": [
     "static/css/main.6034edfe.css",
-    "static/js/main.243399c3.js"
+    "static/js/main.16dc004f.js"
   ]
 }