功能(提醒)：统一项目提醒中心与桌面通知链路

增加通用提醒状态、数据库迁移和定时同步，覆盖风险时效、文件回执、项目里程碑、访视窗口与协作申请。

新增网页端和桌面端提醒中心、真实投递诊断与固定隐私通知正文，并补齐登录来源聚合、测试和说明文档。

backend/app/models/notification.py

@@ -3,7 +3,7 @@ from __future__ import annotations
 import uuid
 from datetime import datetime
 
-from sqlalchemy import DateTime, ForeignKey, Index, String, Text, UniqueConstraint, func
+from sqlalchemy import Boolean, DateTime, ForeignKey, Index, String, Text, UniqueConstraint, func
 from sqlalchemy.dialects.postgresql import UUID
 from sqlalchemy.orm import Mapped, mapped_column
 
@@ -41,6 +41,10 @@ class Notification(Base):
     source_id: Mapped[str] = mapped_column(String(100), nullable=False)
     source_version: Mapped[str | None] = mapped_column(String(100), nullable=True)
     dedupe_key: Mapped[str] = mapped_column(String(255), nullable=False)
+    requires_action: Mapped[bool] = mapped_column(
+        Boolean, nullable=False, default=True, server_default="true"
+    )
+    due_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)
     read_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)
     resolved_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)
     created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), nullable=False, server_default=func.now())