fix alembic migration

api/alembic/script_helpers.py

@@ -0,0 +1,34 @@
+"""Alembic 迁移共享工具（仅用于 versions/ 下的迁移脚本）。"""
+
+from __future__ import annotations
+
+import sqlalchemy as sa
+from alembic import op
+
+
+def table_exists(table_name: str) -> bool:
+    bind = op.get_bind()
+    return table_name in sa.inspect(bind).get_table_names()
+
+
+def has_column(table_name: str, column_name: str) -> bool:
+    if not table_exists(table_name):
+        return False
+    bind = op.get_bind()
+    columns = sa.inspect(bind).get_columns(table_name)
+    return any(column["name"] == column_name for column in columns)
+
+
+def add_column_if_missing(table_name: str, column: sa.Column) -> bool:
+    """若列不存在则 add_column；返回是否执行了添加。"""
+    if has_column(table_name, column.name):
+        return False
+    op.add_column(table_name, column)
+    return True
+
+
+def drop_column_if_exists(table_name: str, column_name: str) -> bool:
+    if not has_column(table_name, column_name):
+        return False
+    op.drop_column(table_name, column_name)
+    return True