feat: trace video shows frame number overlay

src/api/media_api.rs

@@ -310,6 +310,13 @@ async fn trace_video(
 
     // Build filters: bbox+drawtext (1 filter + 1 drawtext per detection)
     let mut parts: Vec<String> = Vec::new();
+
+    // Global frame number overlay (top-left corner, always visible)
+    let global_frame_text = format!(
+        "drawtext=text='Frame\\: %{{n}}+{}':fontsize=16:fontcolor=white:box=1:boxcolor=black@0.6:x=10:y=10",
+        first_frame as i64 - (padding * fps) as i64
+    );
+    parts.push(global_frame_text);
     for (i, (frame, x, y, w, h)) in rows.iter().enumerate() {
         let start_off = frame - first_frame + pad_frames;
         // End at next detection, or at the last frame of the video (not beyond)