feat: 实现站内通知系统

- 后端: 新增 notifications/user_notifications 表迁移及 ORM 模型
- 后端: 实现 schema/repository/service/router 全套通知 API
  - GET /api/v1/notifications (列表+游标分页)
  - GET /api/v1/notifications/unread-count
  - PATCH /api/v1/notifications/{id}/read (幂等)
  - PATCH /api/v1/notifications/mark-all-read (幂等)
- 后端: payload 使用 Pydantic discriminated union (none/open_route/open_url)
- 后端: 19 个单元测试全部通过
- Flutter: 通知 feature 完整实现 (models/apis/repositories/bloc/UI)
- Flutter: Home 页通知按钮接入真实页面，显示未读 badge
- Flutter: 14 个测试全部通过
- 协议文档: notification-inbox-protocol.md 及错误码注册

backend/src/models/user_notification.py

@@ -0,0 +1,41 @@
+from __future__ import annotations
+
+import uuid
+from datetime import datetime
+
+from sqlalchemy import Boolean, DateTime, ForeignKey, UniqueConstraint, text
+from sqlalchemy.dialects.postgresql import UUID
+from sqlalchemy.orm import Mapped, mapped_column
+
+from core.db.base import Base, TimestampMixin
+
+
+class UserNotification(TimestampMixin, Base):
+    __tablename__ = "user_notifications"
+    __table_args__ = (
+        UniqueConstraint(
+            "user_id",
+            "notification_id",
+            name="uq_user_notifications_user_notification",
+        ),
+    )
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True), primary_key=True, default=uuid.uuid4
+    )
+    user_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("auth.users.id", ondelete="CASCADE"),
+        nullable=False,
+    )
+    notification_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("notifications.id", ondelete="CASCADE"),
+        nullable=False,
+    )
+    is_read: Mapped[bool] = mapped_column(
+        Boolean, nullable=False, server_default=text("false")
+    )
+    read_at: Mapped[datetime | None] = mapped_column(
+        DateTime(timezone=True), nullable=True
+    )