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).

frontend/src/views/reports/ReportsIndex.vue

@@ -220,6 +220,7 @@
 import { ref, computed, onMounted, watch } from 'vue'
 import { useRouter, useRoute } from 'vue-router'
 import { reportsApi, businessunitsApi, assetsApi, locationsApi, applicationsApi } from '@/api'
+import { withBase } from '@/utils/basePath'
 
 const router = useRouter()
 const route = useRoute()
@@ -418,7 +419,7 @@ async function runReport(report) {
 function exportCSV() {
   if (!currentReport.value) return
   const params = new URLSearchParams({ format: 'csv', ...filterParams() })
-  window.open(`/api/reports/${currentReport.value.id}?${params}`, '_blank')
+  window.open(withBase(`/api/reports/${currentReport.value.id}?${params}`), '_blank')
 }
 
 function clearReport() {