feat(data): B1-2 指数成分同步（Provider + CLI sync index_weight）

- MarketDataProvider.get_index_weight（协议）；Tushare 实现 normalize_index_weight +
  get_index_weight（ts_code=... 全历史成分权重）；Sina 抛 DataSourceNotSupported；
  FailoverProvider 代理并审计每次尝试
- CLI：sync index_weight --code 000300.SH（拉取→幂等落库 index_weight→打印最新快照；
  失败走 sync_log 审计并返回非零）
- tests：Tushare 映射与调用（FakePro）、Failover 主源单源语义（新浪不支持被审计）；
  全量 pytest 通过

backend/tests/test_failover.py

@@ -91,3 +91,36 @@ class TestFailover:
         with pytest.raises(DataSourceError, match="无备用源"):
             f.get_daily("600519.SH", date(2024, 1, 1), date(2024, 1, 5))
         assert len(collected) == 1
+
+
+class TestIndexFailover:
+    def test_primary_only_index(self) -> None:
+        """指数成分：主源成功即用（新浪不支持会自动尝试并审计失败 → 结果仍来自主源）。"""
+        from datetime import date
+        from decimal import Decimal
+
+        from app.domain.entities.index import IndexWeight
+        from app.infrastructure.data_sources.failover import FailoverProvider
+        from app.infrastructure.data_sources.sina import SinaProvider
+        from app.infrastructure.data_sources.tushare import TushareProvider
+
+        class _P(TushareProvider):
+            name = "tushare"
+
+            def __init__(self):  # 不触网
+                pass
+
+            def get_index_weight(self, index_code):
+                return [IndexWeight(index_code=index_code, index_name="沪深300",
+                                    trade_date=date(2024, 6, 28), symbol="600519.SH",
+                                    weight=Decimal("1"))]
+
+        logs: list[SyncLog] = []
+
+        def audit(log: SyncLog):
+            logs.append(log)
+
+        f = FailoverProvider(_P(), SinaProvider(), audit=audit)
+        rows = f.get_index_weight("000300.SH")
+        assert len(rows) == 1 and rows[0].symbol == "600519.SH"
+        assert len(logs) >= 1  # 每次尝试已审计