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/main.py

@@ -20,6 +20,7 @@ from app.crud.user import ensure_admin_exists
 from app.db.base import Base
 from app.db.session import SessionLocal, engine
 from app.services.visit_scheduler import run_daily_lost_visit_job
+from app.services.notification_scheduler import run_notification_sync_job
 from app.services.permission_log_writer import start_log_writer, stop_log_writer
 from app.services.permission_metric_aggregator import run_hourly_metric_aggregation
 from app.services.source_location_aggregator import run_hourly_source_location_aggregation
@@ -67,6 +68,7 @@ async def lifespan(_: FastAPI):
             await conn.run_sync(Base.metadata.create_all)
         async with SessionLocal() as session:
             await ensure_admin_exists(session)
+    notification_scheduler_task = asyncio.create_task(run_notification_sync_job(stop_event))
     retention_task = asyncio.create_task(run_monitoring_retention(stop_event))
     yield
     stop_event.set()
@@ -75,6 +77,7 @@ async def lifespan(_: FastAPI):
     await scheduler_task
     await aggregator_task
     await source_location_aggregator_task
+    await notification_scheduler_task
     await retention_task