Support subpath IIS deployment as a second install method

The app can run as an IIS Application under an existing site
(e.g. https://host/ops/) instead of its own site + port:

- frontend: vite base via VITE_BASE_PATH; router history, axios
  baseURL, and root-absolute asset/route paths resolve through
  utils/basePath.js withBase()
- backend: MOUNT_PATH (env or .env) wraps the app in a WSGI
  middleware that shifts the prefix into SCRIPT_NAME, so one knob
  serves API + SPA under the mount
- docs: INSTALL-WINDOWS-IIS.md section 7b runbook + troubleshooting
  rows; DEPLOY-WINDOWS-IIS.md pointer; commented examples in
  deploy/windows/web.config and .env.example

Root deployment unchanged (MOUNT_PATH unset, base '/'). Also folds
two stray root-absolute callers into the shared plumbing
(MachineForm relationship-types fetch, reports CSV window.open).

.env.example

@@ -79,3 +79,10 @@ ZABBIX_TOKEN=
 # CMMC_USB_DB_USER=
 # CMMC_USB_DB_PASSWORD=
 # CMMC_USB_DB_NAME=cmmc_usb
+
+# ---- Subpath deployment (optional) ----
+# Serve the app under a URL prefix instead of the server root, e.g. as an IIS
+# Application at /ops under an existing site. The frontend must be rebuilt with
+# the matching base: VITE_BASE_PATH=/ops/ npm run build. Leave unset when the
+# app owns its own site/port (the default). See docs/INSTALL-WINDOWS-IIS.md.
+# MOUNT_PATH=/ops