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/schemas/notification.py

@@ -37,17 +37,6 @@ class DesktopNotificationClaimRequest(BaseModel):
     limit: int = 20
 
 
-class DesktopNotificationClaimResponse(BaseModel):
-    claim_token: uuid.UUID | None = None
-    lease_expires_at: datetime | None = None
-    items: list[NotificationItem]
-
-
-class DesktopNotificationAckRequest(BaseModel):
-    claim_token: uuid.UUID
-    delivered_ids: list[uuid.UUID]
-
-
 class GeneralNotificationRead(BaseModel):
     id: uuid.UUID
     study_id: uuid.UUID
@@ -59,12 +48,27 @@ class GeneralNotificationRead(BaseModel):
     action_path: str | None = None
     source_type: str
     source_id: str
+    requires_action: bool = True
+    due_at: datetime | None = None
     read_at: datetime | None = None
+    resolved_at: datetime | None = None
     created_at: datetime
 
     model_config = ConfigDict(from_attributes=True)
 
 
+class DesktopNotificationClaimResponse(BaseModel):
+    claim_token: uuid.UUID | None = None
+    lease_expires_at: datetime | None = None
+    items: list[GeneralNotificationRead]
+
+
+class DesktopNotificationAckRequest(BaseModel):
+    claim_token: uuid.UUID
+    delivered_ids: list[uuid.UUID]
+
+
 class GeneralNotificationFeed(BaseModel):
     unread_count: int
+    total_count: int
     items: list[GeneralNotificationRead]