Fix isAdminUserMiddleware ignoring DB errors

[alex-vydrin]

Summary

• Bug: isAdminUserMiddleware in app/routes/session.js calls userDAO.getUserById with a callback that receives (err, user) but never checks err. If the database is down or returns an error, the middleware silently treats the request as "not admin" and redirects to /login instead of surfacing the actual error.
• Fix: Added an if (err) return next(err); check in the callback before evaluating the user, consistent with the error-handling pattern used elsewhere in the same file (e.g., line 267).

Test plan

• Verify that when the DB returns an error, the middleware calls next(err) and Express's error handler is invoked
• Verify that normal admin/non-admin flow still works correctly (admin gets through, non-admin is redirected)
• Run existing test suite (npm test)

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