rebuild(stage-5): establish market data gateway and charts

next/backend/data/providers/base.py

@@ -0,0 +1,24 @@
+from __future__ import annotations
+
+from typing import Protocol
+
+from backend.data.contracts import DataSource, ProviderResult
+
+
+class ProviderError(RuntimeError):
+    pass
+
+
+class MarketDataProvider(Protocol):
+    source: DataSource
+
+    @property
+    def configured(self) -> bool: ...
+
+    def calendar(self, start_date: str, end_date: str) -> ProviderResult: ...
+
+    def entities(self) -> ProviderResult: ...
+
+    def daily(self, entity_type: str, identifier: str, end_date: str) -> ProviderResult: ...
+
+    def minute(self, entity_type: str, identifier: str, trade_date: str) -> ProviderResult: ...