feat(HEL-382): 搭建 datahub 底座和盘后正式数据链路

新增独立 xiaobai-datahub 服务（SQLite WAL、Tushare 盘后发布、/v1 契约和管理后台），不改现站页面与数据链路。

Co-authored-by: Cursor <cursoragent@cursor.com>
Co-authored-by: multica-agent <github@multica.ai>

xiaobai-datahub/datahub/numbers.py

@@ -0,0 +1,23 @@
+from __future__ import annotations
+
+import math
+from typing import Any
+
+
+def finite_number(value: Any, default: float | None = None) -> float | None:
+    """Return a finite float, or default (None means JSON null)."""
+    if value is None or value == "":
+        return default
+    try:
+        number = float(value)
+    except (TypeError, ValueError):
+        return default
+    if not math.isfinite(number):
+        return default
+    return number
+
+
+def round4(value: float | None) -> float | None:
+    if value is None:
+        return None
+    return round(float(value), 4)