Maintenance mode for the web application (static export).

IMPORTANT: `staticwebapp.config.json` now carries PRODUCTION routing — the
GitBook/docs redirects, the legacy path rewrites for old /sessions/{id}-style
deep links, the 404 override, and all security headers (incl. the CSP). Never
delete the file or replace it wholesale; both would take those down with it.
The file's content is guarded by lib/__tests__/swaConfig.guard.test.ts.

To ENABLE maintenance mode, insert this object as the FIRST entry of the
existing "routes" array (SWA evaluates routes in order, so it shadows
everything else and serves the maintenance page for all routes):

    { "route": "/*", "serve": "/maintenance.html", "statusCode": 503 },

To DISABLE maintenance mode, remove exactly that first entry again.

Deploy the change like any web change (push → deploy-web workflow).
