feat(benchmarking): add document filename and delete experiment endpoint

Why:
- Experiment list and detail responses showed raw IDs instead of filenames
- No way to delete experiments from the UI

Changes:
- Add document_filename field to ExperimentDetailResponse
- Enrich list endpoint with filenames and calculated best_strategy from aggregate_metrics
- Add DELETE /experiments/{id} endpoint

Impact:
- API responses now include document_filename for all experiment endpoints
- Frontend can display filenames instead of IDs

src/benchmarking/models.py

@@ -119,6 +119,7 @@ class ExperimentDetailResponse(BaseModel):
     """Returned when retrieving an experiment."""
     id: str
     document_id: str
+    document_filename: str = ""
     benchmark_config: dict
     questions: list[dict]
     per_question: list[dict]

src/benchmarking/routes.py

@@ -154,9 +154,15 @@ async def get_benchmark(experiment_id: str):
     if result is None:
         raise BenchmarkError(f"Experiment not found: {experiment_id}")
 
+    # Resolve document filename
+    from src.storage import sqlite as db
+    doc = db.get_document(result.get("document_id", ""))
+    doc_filename = doc.get("filename", "Unknown") if doc else "Deleted"
+
     return ExperimentDetailResponse(
         id=result["id"],
         document_id=result["document_id"],
+        document_filename=doc_filename,
         benchmark_config=result.get("benchmark_config", {}),
         questions=result.get("questions", []),
         per_question=result.get("per_question", []),
@@ -171,7 +177,35 @@ async def get_benchmark(experiment_id: str):
 @router.get("/experiments")
 async def list_experiments(document_id: str | None = None):
     """List all experiments, optionally filtered by document."""
-    return benchmark_service.list_experiments(document_id=document_id)
+    result = benchmark_service.list_experiments(document_id=document_id)
+    # Enrich with document filenames and best_strategy
+    from src.storage import sqlite as db
+    for item in result.get("items", []):
+        doc = db.get_document(item.get("document_id", ""))
+        item["document_filename"] = doc.get("filename", "Unknown") if doc else "Deleted"
+        # Calculate best_strategy from aggregate_metrics
+        aggs = item.get("aggregate_metrics", {})
+        best_strat, best_score = "N/A", -1
+        for strat, m in aggs.items():
+            score = (m.get("avg_context_relevance", 0) * 0.3 +
+                     m.get("avg_answer_similarity", 0) * 0.4 +
+                     m.get("avg_faithfulness", 0) * 0.3)
+            adjusted = score * (1 - m.get("hallucination_rate", 0))
+            if adjusted > best_score:
+                best_score = adjusted
+                best_strat = strat
+        item["best_strategy"] = best_strat
+    return result
+
+
+@router.delete("/experiments/{experiment_id}")
+async def delete_experiment(experiment_id: str):
+    """Delete an experiment by ID."""
+    from src.storage import sqlite as db
+    deleted = db.delete_experiment(experiment_id)
+    if not deleted:
+        raise BenchmarkError(f"Experiment not found: {experiment_id}")
+    return {"deleted": True, "experiment_id": experiment_id}
 
 
 @router.get("/benchmarks/{experiment_id}/report", response_class=HTMLResponse)