feat: 产品协同留言板 — ProductMessage 模型 + API + 迁移

- 新增 ProductMessage 模型，挂载在 products.id（ON DELETE CASCADE）
- GET/POST /products/{id}/messages 接口
- 注册到 Alembic 自动发现

backend/alembic/versions/g1h2i3j4k5l6_add_product_messages.py

@@ -0,0 +1,33 @@
+"""add_product_messages
+
+Revision ID: g1h2i3j4k5l6
+Revises: c9d0e1f2a3b4
+Create Date: 2026-08-10
+
+"""
+from typing import Sequence, Union
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects import postgresql
+
+revision: str = "g1h2i3j4k5l6"
+down_revision: Union[str, None] = "c9d0e1f2a3b4"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "product_messages",
+        sa.Column("id", postgresql.UUID(as_uuid=True), primary_key=True),
+        sa.Column("product_id", postgresql.UUID(as_uuid=True),
+                  sa.ForeignKey("products.id", ondelete="CASCADE"),
+                  index=True, nullable=False),
+        sa.Column("operator_id", sa.String(50), nullable=False),
+        sa.Column("content", sa.Text, nullable=False),
+        sa.Column("created_at", sa.DateTime, nullable=True),
+    )
+
+
+def downgrade() -> None:
+    op.drop_table("product_messages")