fix: suppress Rust dead code warnings with annotations

- ExportEvent::Error and DownloadEvent::Error reserved for future use
- clip_exporter functions scaffolded for caption burn-in pipeline
- SubtitleFormat fields required for serde deserialization

Co-authored-by: Cursor <cursoragent@cursor.com>

src-tauri/src/commands/video.rs

@@ -20,6 +20,7 @@ pub enum DownloadEvent {
     Progress { percent: f64 },
     FilePath { path: String },
     Finished { success: bool, path: String },
+    #[allow(dead_code)] // Reserved for future error-channel reporting
     Error { message: String },
 }