fix(media-proxy): allow the desktop app origin and preflight (#3325)

fluxer_media_proxy/src/constants.rs

@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: AGPL-3.0-or-later
 
 pub const MAX_MEDIA_PROXY_BYTES: usize = 500 * 1024 * 1024;
+pub const DESKTOP_APP_ORIGIN: &str = "fluxer-app://app";
 
 pub const OUTBOUND_USER_AGENT: &str =
     "Mozilla/5.0 (compatible; Fluxerbot/1.0; +https://fluxer.app)";