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/app/models/message.py

@@ -0,0 +1,32 @@
+"""产品协同留言板模型"""
+import uuid
+from datetime import datetime
+from sqlalchemy import String, Text, DateTime, ForeignKey
+from sqlalchemy.dialects.postgresql import UUID
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.models.base import Base
+
+
+class ProductMessage(Base):
+    __tablename__ = "product_messages"
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True), primary_key=True, default=uuid.uuid4,
+    )
+    product_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("products.id", ondelete="CASCADE"),
+        index=True,
+        nullable=False,
+        comment="所属产品 ID",
+    )
+    operator_id: Mapped[str] = mapped_column(
+        String(50), nullable=False, comment="留言人姓名或工号",
+    )
+    content: Mapped[str] = mapped_column(
+        Text, nullable=False, comment="留言内容",
+    )
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime, default=datetime.utcnow, comment="留言时间",
+    )