feat(token-tracking): surface token_usage in ReportData

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

webapp/services/report_builder.py

@@ -187,6 +187,9 @@ def build_report(run_dir: Path, metrics: list[str]) -> ReportData:
     summary_markdown = run_reader.read_summary_markdown(run_dir)
     advice_markdown = run_reader.read_advice_markdown(run_dir)
     metric_weights, doc_weights = _read_weights_from_snapshot(run_dir)
+    # Read once up front so both the empty-frame and full branches can surface it.
+    metadata = run_reader._read_json(run_dir / "metadata.json")
+    token_usage = metadata.get("token_usage") or {}
 
     if frame.empty or not metrics:
         return ReportData(
@@ -196,6 +199,7 @@ def build_report(run_dir: Path, metrics: list[str]) -> ReportData:
             advice_markdown=advice_markdown,
             metric_weights=metric_weights,
             doc_weights=doc_weights,
+            token_usage=token_usage,
         )
 
     score_rows_list = frame.to_dict(orient="records")
@@ -218,7 +222,6 @@ def build_report(run_dir: Path, metrics: list[str]) -> ReportData:
 
     # Cross-run history: scores of the same question in *other* runs (Approach A —
     # on-demand global scan, excluding the run currently being viewed).
-    metadata = run_reader._read_json(run_dir / "metadata.json")
     current_run_id = str(metadata.get("run_id") or run_dir.name)
     history_index = question_history.build_question_history_index(
         exclude_run_id=current_run_id
@@ -235,4 +238,5 @@ def build_report(run_dir: Path, metrics: list[str]) -> ReportData:
         weighted_score_mean=_round_or_none(overall_ws),
         metric_weights=metric_weights,
         doc_weights=doc_weights,
+        token_usage=token_usage,
     )