feat: add Streamable downloader endpoint + fix detect false-positive

- /download/streamable: real MP4 via yt-dlp (verified ftypisom)
- Fix detect_platform: t.co substring greedily matched reddit.com
  ('.reddit.com' contains 't.co') - now requires t.co/ path
- Removed duplicate reddit branch in detect chain

src/presentation/router.rs

@@ -244,6 +244,10 @@ pub fn build_router(app_state: Arc<AppState>) -> anyhow::Result<Router> {
             "/download/reddit",
             axum::routing::get(crate::presentation::handler::downloader::download_reddit),
         )
+        .route(
+            "/download/streamable",
+            axum::routing::get(crate::presentation::handler::downloader::download_streamable),
+        )
         .route(
             "/download/bilibili",
             axum::routing::get(crate::presentation::handler::downloader::download_bilibili),