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/application/mod.rs

@@ -1,4 +1,5 @@
 pub mod anime;
 pub mod anime2;
+pub mod downloader;
 pub mod komik;
 pub mod proxy;