feat(backend): add websocket schedule operations

[yyy-router]

变更说明

• 新增日程业务服务，支持创建、更新、类型校验和时间冲突提示
• 新增 SQLAlchemy 日程仓储，实现查询、保存和冲突检测
• 实现 schedule.upsert.command WebSocket 消息处理
• 实现 schedule.list.query WebSocket 消息处理
• 在应用组合根注册日程消息 handler
• 补充业务服务、仓储和 WebSocket handler 测试

验证结果

• uv run ruff check .
• uv run mypy
• uv run pytest -q，共 80 个测试通过
• 已完成 WebSocket 连接、日程创建更新和列表查询端到端测试

Closes #107

[fennoai]

Review summary

Found three issues affecting schedule isolation, input error handling, and chronological ordering.

Verification: python3 -m compileall -q backend/src backend/tests and git diff --check origin/MVP...HEAD passed. The project test/lint/type-check commands could not run because uv is unavailable in this environment.

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