feat(工作日): 新增工作小时计算工具 + 节假日表与管理接口

- time_utils 新增 to_beijing(统一naive按UTC转北京时间) + working_duration_hours(排除周末/节假日)
- Holiday 模型 + alembic 迁移建 holidays 表
- GET/POST/DELETE /holidays 节假日管理接口，可随时配置放假日期

backend/alembic/versions/h1h2h3h4h5h6_add_holidays.py

@@ -0,0 +1,32 @@
+"""add holidays table
+
+Revision ID: h1h2h3h4h5h6
+Revises: g1h2i3j4k5l6
+Create Date: 2026-08-28 12:00:00.000000
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+revision: str = 'h1h2h3h4h5h6'
+down_revision: Union[str, Sequence[str], None] = 'g1h2i3j4k5l6'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        'holidays',
+        sa.Column('id', sa.Integer(), autoincrement=True, primary_key=True),
+        sa.Column('day', sa.Date(), nullable=False, comment='放假日期'),
+        sa.Column('name', sa.String(length=100), nullable=True, comment='放假说明（如国庆节）'),
+    )
+    op.create_index('ix_holidays_day', 'holidays', ['day'], unique=True)
+
+
+def downgrade() -> None:
+    op.drop_index('ix_holidays_day', table_name='holidays')
+    op.drop_table('holidays')