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).

wsgi.py

@@ -10,5 +10,36 @@ from shopdb import create_app
 
 app = create_app(os.environ.get('FLASK_ENV', 'development'))
 
+
+class MountPathMiddleware:
+    """Serve the whole app (API + SPA) under a URL prefix, e.g. '/ops'.
+
+    Used when the app is deployed as an IIS Application under an existing
+    site instead of its own site: IIS forwards the full request path
+    ('/ops/api/...'), so the prefix is moved from PATH_INFO to SCRIPT_NAME
+    before Flask routes it. Flask then also generates URLs under the prefix.
+    The frontend must be built with the matching VITE_BASE_PATH ('/ops/').
+    """
+
+    def __init__(self, wsgi_app, mountpath):
+        self.wsgi_app = wsgi_app
+        self.mountpath = '/' + mountpath.strip('/')
+
+    def __call__(self, environ, start_response):
+        path = environ.get('PATH_INFO', '')
+        if path == self.mountpath or path.startswith(self.mountpath + '/'):
+            environ['SCRIPT_NAME'] = environ.get('SCRIPT_NAME', '') + self.mountpath
+            environ['PATH_INFO'] = path[len(self.mountpath):] or '/'
+            return self.wsgi_app(environ, start_response)
+        start_response('404 Not Found', [('Content-Type', 'text/plain')])
+        return [b'Not Found: the app is mounted at ' + self.mountpath.encode() + b'/']
+
+
+# MOUNT_PATH (.env or web.config) activates the subpath deployment method.
+# Unset/empty = the app owns the server root (its own IIS site; the default).
+_mountpath = os.environ.get('MOUNT_PATH', '').strip()
+if _mountpath and _mountpath != '/':
+    app.wsgi_app = MountPathMiddleware(app.wsgi_app, _mountpath)
+
 if __name__ == '__main__':
     app.run(host='0.0.0.0', port=5001, debug=True)