feat(api): 新增核心 API 端点 + 数据库迁移

新增 3 个 API 端点:
- POST /tasks/{id}/receive: 确认接收 (PENDING→WIP)
- POST /tasks/{id}/reject: 品质驳回 + 返工闭环
- POST /tasks/{id}/transfer: 完工裂变转交 (多路分支+入库)

数据库迁移 (a1b2c3d4e5f6):
- tasks 表新增 received_at, completed_at, reject_reason, is_rework
- products 表新增 current_location_id
- 数据迁移: 已有任务状态小写→大写 (pending→PENDING 等)

backend/alembic/versions/a1b2c3d4e5f6_add_task_rework_and_product_location.py

@@ -0,0 +1,102 @@
+"""add task rework fields and product current_location
+
+Revision ID: a1b2c3d4e5f6
+Revises: 80f8cb64544a
+Create Date: 2026-08-04 12:00:00.000000
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = 'a1b2c3d4e5f6'
+down_revision: Union[str, Sequence[str], None] = '80f8cb64544a'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    """Upgrade schema."""
+    # === 数据迁移：将已有任务状态从小写转为大写 ===
+    op.execute("""
+        UPDATE tasks SET status = 'PENDING'   WHERE lower(status) = 'pending';
+    """)
+    op.execute("""
+        UPDATE tasks SET status = 'WIP'       WHERE lower(status) = 'wip';
+    """)
+    op.execute("""
+        UPDATE tasks SET status = 'COMPLETED' WHERE lower(status) = 'completed';
+    """)
+    op.execute("""
+        UPDATE tasks SET status = 'REJECTED'  WHERE lower(status) = 'rejected';
+    """)
+    op.execute("""
+        UPDATE tasks SET status = 'ARCHIVED'  WHERE lower(status) = 'archived';
+    """)
+
+    # === tasks 表新增字段 ===
+    op.add_column('tasks', sa.Column(
+        'received_at',
+        sa.DateTime(timezone=True),
+        nullable=True,
+        comment='操作员确认接收时间',
+    ))
+    op.add_column('tasks', sa.Column(
+        'completed_at',
+        sa.DateTime(timezone=True),
+        nullable=True,
+        comment='任务完工转交时间',
+    ))
+    op.add_column('tasks', sa.Column(
+        'reject_reason',
+        sa.String(length=500),
+        nullable=True,
+        comment='驳回原因',
+    ))
+    op.add_column('tasks', sa.Column(
+        'is_rework',
+        sa.Boolean(),
+        nullable=False,
+        server_default=sa.text('false'),
+        comment='是否为返工任务',
+    ))
+
+    # === products 表新增字段 ===
+    op.add_column('products', sa.Column(
+        'current_location_id',
+        sa.String(length=64),
+        nullable=True,
+        comment="当前持有者ID 或 'virtual_warehouse'(仓库)",
+    ))
+
+
+def downgrade() -> None:
+    """Downgrade schema."""
+    # === products 表移除字段 ===
+    op.drop_column('products', 'current_location_id')
+
+    # === tasks 表移除字段 ===
+    op.drop_column('tasks', 'is_rework')
+    op.drop_column('tasks', 'reject_reason')
+    op.drop_column('tasks', 'completed_at')
+    op.drop_column('tasks', 'received_at')
+
+    # === 数据回迁：将任务状态从大写转回小写 ===
+    op.execute("""
+        UPDATE tasks SET status = 'pending'   WHERE status = 'PENDING';
+    """)
+    op.execute("""
+        UPDATE tasks SET status = 'wip'       WHERE status = 'WIP';
+    """)
+    op.execute("""
+        UPDATE tasks SET status = 'completed' WHERE status = 'COMPLETED';
+    """)
+    op.execute("""
+        UPDATE tasks SET status = 'rejected'  WHERE status = 'REJECTED';
+    """)
+    op.execute("""
+        UPDATE tasks SET status = 'archived'  WHERE status = 'ARCHIVED';
+    """)