v2.1: 新增通知提醒功能和安全增强

功能:
- 通知提醒系统: 还款日提醒、逾期提醒、系统消息横幅、登录时通知
- 通知中心UI: 铃铛图标 + 下拉列表 + 未读徽章
- 定时任务: APScheduler 每小时检查即将到期的还款
- 邮件通知: SMTP 发送还款提醒和安全警告
- 异常登录通知: 3/5/10次失败触发网页和邮件通知

安全:
- 登录安全告警: 异常登录次数达到阈值时发送通知
- 密码重置链接: 一次性使用, 5分钟有效期
- 登录频率限制: IP级和账户级限制

技术:
- 后端: FastAPI + SQLAlchemy + PostgreSQL
- 前端: 纯 HTML/CSS/JS
- 部署: Docker Compose (新增 build 配置)

backend/app/services/notification_service.py

@@ -0,0 +1,41 @@
+from sqlalchemy import select
+from sqlalchemy.ext.asyncio import AsyncSession
+from app.models.models import Notification, NotificationType, User
+
+
+async def create_notification(
+    db: AsyncSession,
+    user_id: int,
+    title: str,
+    message: str,
+    notification_type: str = NotificationType.SYSTEM.value,
+    related_debt_id: int | None = None,
+    related_plan_id: int | None = None,
+) -> Notification:
+    n = Notification(
+        user_id=user_id,
+        title=title,
+        message=message,
+        type=notification_type,
+        related_debt_id=related_debt_id,
+        related_plan_id=related_plan_id,
+    )
+    db.add(n)
+    return n
+
+
+async def create_system_broadcast(
+    db: AsyncSession,
+    title: str,
+    message: str,
+) -> int:
+    users = await db.execute(select(User).where(User.status == "active"))
+    count = 0
+    for user in users.scalars().all():
+        db.add(Notification(
+            user_id=user.id, title=title, message=message,
+            type=NotificationType.SYSTEM.value,
+        ))
+        count += 1
+    await db.commit()
+    return count