Fix ERPNext and Forgejo probes

- ERPNext: custom blackbox module with Host: performancewest.net header
  (ERPNext multitenancy requires site name in Host for routing)
- Forgejo: add extra_hosts to blackbox-exporter so it can resolve
  host.docker.internal to reach forgejo on port 3000
- Blackbox http_erpnext module: sets Host header, expects 200

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

monitoring/blackbox.yml

@@ -20,6 +20,17 @@ modules:
       follow_redirects: false
       preferred_ip_protocol: ip4
 
+  # ERPNext requires Host header matching the site name
+  http_erpnext:
+    prober: http
+    timeout: 10s
+    http:
+      valid_http_versions: ["HTTP/1.1", "HTTP/2.0"]
+      valid_status_codes: [200]
+      headers:
+        Host: performancewest.net
+      preferred_ip_protocol: ip4
+
   tcp_connect:
     prober: tcp
     timeout: 5s