GET /api/rooms/export returns 200 with an empty error.csv instead of 403

Steps to reproduce

1. Log in as a user who is not a Nextcloud administrator.
2. Request GET /apps/roomvox/api/rooms/export.

Expected behaviour

403 Forbidden, consistent with every other admin-only endpoint in the app.

Actual behaviour

200 OK with a CSV download named error.csv and an empty body.

Cause

https://github.com/nextcloud/RoomVox/blob/main/lib/Controller/RoomApiController.php#L539-L543

$userId = $this->getCurrentUserId();
if ($userId === null || !$this->groupManager->isAdmin($userId)) {
    return new DataDownloadResponse('', 'error.csv', 'text/csv');
}

The permission check is correct — the response to a failed check is not. It
signals the refusal through a filename instead of a status code.

Why this matters

No data leaks: the body is empty, so this is a correctness and consistency
issue rather than a security hole.

What it does break is anything that checks the status code, which is the normal
way to handle an HTTP error. A script that pipes this endpoint into a file gets
a zero-byte CSV and a success status, so a permissions problem looks like "there
are no rooms". The same applies to a non-admin hitting a stale browser tab: an
empty file downloads and nothing explains why.

It is also inconsistent within the same controller — import and
importPreview next to it both return a proper 403 with
{"error": "Admin access required"}.

Suggested fix

Return a real error response, as the neighbouring endpoints do. Note the return
type is DataDownloadResponse, so the signature has to widen (Response, or a
JSONResponse union) to return a 403:

if ($userId === null || !$this->groupManager->isAdmin($userId)) {
    return new JSONResponse(['error' => 'Admin access required'], Http::STATUS_FORBIDDEN);
}

Worth checking at the same time: GET /api/rooms/sample-csv
(RoomApiController.php:555) performs no check of its own at all. It is still
admin-gated, because Nextcloud requires an admin session for any controller
method not marked #[NoAdminRequired], so this is not a hole either — but the
protection is implicit where every endpoint around it is explicit.

Notes

Found while auditing docs/architecture/api-reference.md against the code. The
current behaviour is now documented so the reference is accurate, and should be
updated again once this is fixed.

Start in lib/Controller/RoomApiController.php around lines 539-543 and compare the export response with the neighbouring import and importPreview endpoints. Verify the non-admin and unauthenticated cases return a 403 with the stated JSON error rather than an empty CSV, then check docs/architecture/api-reference.md for the behavior that needs updating.