fix(tkg): handle null identity_id + remove skin_tone nodes

- Fix Phase 2.5 null handling in build_gaze/lip_track_nodes
  - Use query_scalar::<_, Option<i64>> + flatten() for nullable fields
  - Prevents 'unexpected null' decoding errors

- Remove skin_tone_trace_nodes from TKG build
  - Delete build_skin_tone_trace_nodes function (110 lines)
  - Remove from TkgResult struct and API response
  - Skin tone should be independent function, not in TKG

Result: TKG rebuild now completes successfully
- Nodes: 40 (face_track, gaze_track, text_region, appearance)
- Edges: 2967 (co_occurrence edges increased from 21 → 2964)

src/api/trace_agent_api.rs

@@ -997,13 +997,12 @@ async fn rebuild_tkg(
                 success: true,
                 file_uuid,
                 result: Some(serde_json::json!({
-                    "face_track_nodes": r.face_track_nodes,
-                    "gaze_track_nodes": r.gaze_track_nodes,
-                    "lip_track_nodes": r.lip_track_nodes,
-                    "text_region_nodes": r.text_region_nodes,
-                    "appearance_trace_nodes": r.appearance_trace_nodes,
-                    "skin_tone_trace_nodes": r.skin_tone_trace_nodes,
-                    "accessory_nodes": r.accessory_nodes,
+"face_track_nodes": r.face_track_nodes,
+"gaze_track_nodes": r.gaze_track_nodes,
+"lip_track_nodes": r.lip_track_nodes,
+"text_region_nodes": r.text_region_nodes,
+"appearance_trace_nodes": r.appearance_trace_nodes,
+"accessory_nodes": r.accessory_nodes,
                     "object_nodes": r.object_nodes,
                     "speaker_nodes": r.speaker_nodes,
                     "co_occurrence_edges": r.co_occurrence_edges,