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/services/notification_scheduler.py

@@ -0,0 +1,78 @@
+"""Server-side materialization for time and state driven business reminders."""
+
+from __future__ import annotations
+
+import asyncio
+import logging
+
+from sqlalchemy import select, text
+
+from app.core.config import settings
+from app.db.session import SessionLocal
+from app.models.study_member import StudyMember
+from app.models.user import User, UserStatus
+from app.services.project_reminder_service import sync_project_reminders
+
+logger = logging.getLogger("ctms.notifications")
+NOTIFICATION_SYNC_LOCK_KEY = 0x43544D53  # "CTMS"
+
+
+async def sync_all_project_reminders_once() -> int:
+    """Synchronize every active project member once, with a cross-worker advisory lock."""
+    async with SessionLocal() as lock_session:
+        acquired = bool(await lock_session.scalar(
+            text("SELECT pg_try_advisory_lock(:lock_key)"),
+            {"lock_key": NOTIFICATION_SYNC_LOCK_KEY},
+        ))
+        if not acquired:
+            return 0
+        try:
+            memberships = (await lock_session.execute(
+                select(StudyMember.study_id, StudyMember.user_id)
+                .join(User, User.id == StudyMember.user_id)
+                .where(
+                    StudyMember.is_active.is_(True),
+                    User.status == UserStatus.ACTIVE,
+                )
+            )).all()
+            await lock_session.commit()
+            synced = 0
+            for study_id, user_id in memberships:
+                async with SessionLocal() as session:
+                    try:
+                        user = await session.get(User, user_id)
+                        if user is None or not user.is_active:
+                            continue
+                        await sync_project_reminders(session, study_id, user)
+                        synced += 1
+                    except Exception:
+                        await session.rollback()
+                        logger.exception(
+                            "Failed to synchronize reminders for study=%s user=%s",
+                            study_id,
+                            user_id,
+                        )
+            return synced
+        finally:
+            await lock_session.execute(
+                text("SELECT pg_advisory_unlock(:lock_key)"),
+                {"lock_key": NOTIFICATION_SYNC_LOCK_KEY},
+            )
+
+
+async def run_notification_sync_job(stop_event: asyncio.Event) -> None:
+    logger.info("Notification synchronization task started")
+    while not stop_event.is_set():
+        try:
+            synced = await sync_all_project_reminders_once()
+            logger.debug("Synchronized reminders for %s project memberships", synced)
+        except Exception:
+            logger.exception("Notification synchronization task failed")
+        try:
+            await asyncio.wait_for(
+                stop_event.wait(),
+                timeout=settings.NOTIFICATION_SYNC_INTERVAL_SECONDS,
+            )
+        except asyncio.TimeoutError:
+            continue
+    logger.info("Notification synchronization task stopped")