feat: add metric_weights and doc_weights to Scenario schema and dataclass

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

rag_eval/shared/models.py

@@ -77,6 +77,8 @@ class Scenario:
     app_adapter: AppAdapterConfig | None = None
     source_path: Path | None = None
     optimization_advisor: bool = False
+    metric_weights: dict[str, float] = field(default_factory=dict)
+    doc_weights: dict[str, float] = field(default_factory=dict)
 
     def snapshot(self) -> dict[str, Any]:
         """Serialize the scenario into a reporting-friendly dictionary snapshot."""