feat: save key_frame.jpg and key_face.jpg for each face trace during TKG build

- Extract full frame at best confidence frame using ffmpeg
- Crop face region using average bbox coordinates
- Save to .faces/{file_uuid}/{trace_id}/key_frame.jpg and key_face.jpg
- TKG node properties store full file paths for UI display
- Supports future extension for cut representative frames

src/core/processor/tkg.rs

@@ -1218,6 +1218,69 @@ async fn build_face_track_nodes_from_qdrant(
             "status": "pending",
         });
 
+        // Save key frame and key face JPGs
+        let output_dir = crate::core::config::OUTPUT_DIR.as_str();
+        let face_dir = std::path::PathBuf::from(output_dir)
+            .join(".faces").join(file_uuid).join(&external_id);
+        let _ = std::fs::create_dir_all(&face_dir);
+
+        let key_frame_path = face_dir.join("key_frame.jpg");
+        let key_face_path = face_dir.join("key_face.jpg");
+
+        // Get video path from database
+        let video_path: Option<String> = sqlx::query_scalar(&format!(
+            "SELECT file_path FROM {} WHERE file_uuid = $1",
+            crate::core::db::schema::table_name("videos")
+        ))
+        .bind(file_uuid)
+        .fetch_optional(pool)
+        .await
+        .ok()
+        .flatten();
+
+        // Extract key frame using ffmpeg
+        if !key_frame_path.exists() {
+            if let Some(ref vp) = video_path {
+                let timestamp = agg.best_frame as f64 / 30.0;
+                let _ = std::process::Command::new("ffmpeg")
+                    .args(["-y", "-v", "quiet", "-ss", &timestamp.to_string(), "-i", vp])
+                    .args(["-frames:v", "1", "-q:v", "2"])
+                    .arg(key_frame_path.to_str().unwrap_or(""))
+                    .output();
+            }
+        }
+
+        // Crop key face from key frame using bbox
+        if !key_face_path.exists() && key_frame_path.exists() {
+            let bbox = &props["avg_bbox"];
+            let x = bbox["x"].as_i64().unwrap_or(0) as u32;
+            let y = bbox["y"].as_i64().unwrap_or(0) as u32;
+            let w = bbox["width"].as_i64().unwrap_or(100) as u32;
+            let h = bbox["height"].as_i64().unwrap_or(100) as u32;
+            let _ = std::process::Command::new("ffmpeg")
+                .args(["-y", "-v", "quiet", "-i", key_frame_path.to_str().unwrap_or("")])
+                .args(["-vf", &format!("crop={}:{}:{}:{}", w, h, x, y)])
+                .args(["-q:v", "2"])
+                .arg(key_face_path.to_str().unwrap_or(""))
+                .output();
+        }
+
+        // Update props with actual file paths
+        let final_props = serde_json::json!({
+            "trace_id": tid,
+            "frame_count": agg.frame_count,
+            "start_frame": agg.start_f,
+            "end_frame": agg.end_f,
+            "key_frame": if key_frame_path.exists() { key_frame_path.to_string_lossy().to_string() } else { serde_json::Value::Null.to_string() },
+            "key_face": if key_face_path.exists() { key_face_path.to_string_lossy().to_string() } else { serde_json::Value::Null.to_string() },
+            "avg_bbox": props["avg_bbox"].clone(),
+            "avg_yaw": props["avg_yaw"].clone(),
+            "avg_pitch": props["avg_pitch"].clone(),
+            "avg_roll": props["avg_roll"].clone(),
+            "pose_count": pose_count,
+            "status": "pending",
+        });
+
         sqlx::query(&format!(
             r#"
             INSERT INTO {} (node_type, external_id, file_uuid, label, properties)
@@ -1232,7 +1295,7 @@ async fn build_face_track_nodes_from_qdrant(
         .bind(&external_id)
         .bind(file_uuid)
         .bind(&label)
-        .bind(serde_json::to_string(&props)?)
+        .bind(serde_json::to_string(&final_props)?)
         .execute(pool)
         .await?;