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/TVDashboard.vue

@@ -7,7 +7,7 @@
         class="slide"
         :class="{ active: idx === currentSlide }"
       >
-        <img :src="basePath + slide.filename" :alt="slide.filename" />
+        <img :src="withBase(basePath + slide.filename)" :alt="slide.filename" />
       </div>
 
       <div v-if="error" class="error-message">
@@ -28,6 +28,7 @@
 <script setup>
 import { ref, computed, onMounted, onUnmounted } from 'vue'
 import api from '@/api'
+import { withBase } from '@/utils/basePath'
 
 const INTERVAL = 10 // seconds between slides