feat(downloader): add 26 downloader APIs + detect endpoint

Port downloaders from Shirokami-API (Node.js):
- Instagram/Facebook (snapsave.app, downr.org fallback)
- TikTok (tikwm.com)
- YouTube MP4/MP3 (savetube.media, ydlp.yard.id)
- Spotify, Twitter/X (api.lrm.tube)
- Bilibili, Pinterest, MediaFire, Mega, TeraBox
- PixelDrain, Threads, DoodStream, KrakenFiles
- Danbooru, SoundCloud, Google Drive

Domain layer: DownloadResult + MediaItem entities
Application layer: 7 use-cases + detect_platform auto-detect
Infrastructure: DownloaderRepository (22 fetch functions)
Presentation: 21 handlers + DTO + OpenAPI schemas

Also: upgrade OpenTelemetry to 0.28 (axum 0.7 conflict resolution)

src/presentation/router.rs

@@ -155,6 +155,91 @@ pub fn build_router(app_state: Arc<AppState>) -> anyhow::Result<Router> {
         )
         // Proxy routes
         // (all proxy routes removed)
+        // Downloader routes
+        .route(
+            "/download",
+            axum::routing::get(crate::presentation::handler::downloader::download),
+        )
+        .route(
+            "/download/detect",
+            axum::routing::get(crate::presentation::handler::downloader::detect_platform_handler),
+        )
+        .route(
+            "/download/instagram",
+            axum::routing::get(crate::presentation::handler::downloader::download_instagram),
+        )
+        .route(
+            "/download/facebook",
+            axum::routing::get(crate::presentation::handler::downloader::download_facebook),
+        )
+        .route(
+            "/download/tiktok",
+            axum::routing::get(crate::presentation::handler::downloader::download_tiktok),
+        )
+        .route(
+            "/download/youtube",
+            axum::routing::get(crate::presentation::handler::downloader::download_youtube),
+        )
+        .route(
+            "/download/youtube/mp3",
+            axum::routing::get(crate::presentation::handler::downloader::download_youtube_mp3),
+        )
+        .route(
+            "/download/spotify",
+            axum::routing::get(crate::presentation::handler::downloader::download_spotify),
+        )
+        .route(
+            "/download/twitter",
+            axum::routing::get(crate::presentation::handler::downloader::download_twitter),
+        )
+        .route(
+            "/download/pinterest",
+            axum::routing::get(crate::presentation::handler::downloader::download_pinterest),
+        )
+        .route(
+            "/download/mega",
+            axum::routing::get(crate::presentation::handler::downloader::download_mega),
+        )
+        .route(
+            "/download/terabox",
+            axum::routing::get(crate::presentation::handler::downloader::download_terabox),
+        )
+        .route(
+            "/download/gdrive",
+            axum::routing::get(crate::presentation::handler::downloader::download_gdrive),
+        )
+        .route(
+            "/download/mediafire",
+            axum::routing::get(crate::presentation::handler::downloader::download_mediafire),
+        )
+        .route(
+            "/download/pixeldrain",
+            axum::routing::get(crate::presentation::handler::downloader::download_pixeldrain),
+        )
+        .route(
+            "/download/threads",
+            axum::routing::get(crate::presentation::handler::downloader::download_threads),
+        )
+        .route(
+            "/download/dood",
+            axum::routing::get(crate::presentation::handler::downloader::download_doodstream),
+        )
+        .route(
+            "/download/kraken",
+            axum::routing::get(crate::presentation::handler::downloader::download_krakenfiles),
+        )
+        .route(
+            "/download/danbooru",
+            axum::routing::get(crate::presentation::handler::downloader::download_danbooru),
+        )
+        .route(
+            "/download/soundcloud",
+            axum::routing::get(crate::presentation::handler::downloader::download_soundcloud),
+        )
+        .route(
+            "/download/bilibili",
+            axum::routing::get(crate::presentation::handler::downloader::download_bilibili),
+        )
         // Health
         .route(
             "/health",