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/desktop_notification.py

@@ -28,6 +28,7 @@ class DesktopNotificationDelivery(Base):
     __tablename__ = "desktop_notification_deliveries"
     __table_args__ = (
         UniqueConstraint("user_id", "distribution_id", name="uq_desktop_notification_user_distribution"),
+        UniqueConstraint("user_id", "notification_id", name="uq_desktop_notification_user_notification"),
         Index("ix_desktop_notification_claim", "user_id", "delivered_at", "claimed_at"),
     )
 
@@ -35,8 +36,11 @@ class DesktopNotificationDelivery(Base):
     user_id: Mapped[uuid.UUID] = mapped_column(
         UUID(as_uuid=True), ForeignKey("users.id", ondelete="CASCADE"), nullable=False
     )
-    distribution_id: Mapped[uuid.UUID] = mapped_column(
-        UUID(as_uuid=True), ForeignKey("distributions.id", ondelete="CASCADE"), nullable=False
+    distribution_id: Mapped[uuid.UUID | None] = mapped_column(
+        UUID(as_uuid=True), ForeignKey("distributions.id", ondelete="CASCADE"), nullable=True
+    )
+    notification_id: Mapped[uuid.UUID | None] = mapped_column(
+        UUID(as_uuid=True), ForeignKey("notifications.id", ondelete="CASCADE"), nullable=True
     )
     claim_token: Mapped[uuid.UUID | None] = mapped_column(UUID(as_uuid=True), nullable=True)
     claimed_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)