fix: 协同留言时间改为北京时间

根因: ProductMessage.created_at 使用 datetime.utcnow (UTC),
      与其他所有模型使用的 get_beijing_time 不一致。
      看板留言抽屉显示的时间比实际晚8小时。

修复:
  1. message.py: DateTime → DateTime(timezone=True)
     default 从 datetime.utcnow → get_beijing_time
  2. dashboard_service.py: 兜底转换已有数据
     naive UTC → replace(tzinfo=UTC) → astimezone(Beijing)

backend/app/models/message.py

@@ -6,6 +6,7 @@ from sqlalchemy.dialects.postgresql import UUID
 from sqlalchemy.orm import Mapped, mapped_column
 
 from app.models.base import Base
+from app.core.time_utils import get_beijing_time
 
 
 class ProductMessage(Base):
@@ -28,5 +29,5 @@ class ProductMessage(Base):
         Text, nullable=False, comment="留言内容",
     )
     created_at: Mapped[datetime] = mapped_column(
-        DateTime, default=datetime.utcnow, comment="留言时间",
+        DateTime(timezone=True), default=get_beijing_time, comment="留言时间（北京时间）",
     )

backend/app/services/dashboard_service.py

@@ -226,8 +226,13 @@ async def search_product_messages(
     items: list[ProductMessageItem] = []
     for msg, sn, mat_name in rows:
         t = msg.created_at
-        if t and t.tzinfo is None:
-            t = t.replace(tzinfo=BEIJING_TZ)
+        if t:
+            if t.tzinfo is None:
+                # 旧数据（datetime.utcnow）：naive UTC → 转北京时间
+                from datetime import timezone as dt_timezone
+                t = t.replace(tzinfo=dt_timezone.utc).astimezone(BEIJING_TZ)
+            else:
+                t = t.astimezone(BEIJING_TZ)
         time_str = t.isoformat() if t else ""
 
         items.append(ProductMessageItem(