Update wire protocol to version 6 and enhance timetable functionality

- Bumped the wire protocol version to 6, reflecting changes in the communication structure.
- Expanded the timetable API with new endpoints for fetching and managing lesson schedules, including `GET /api/schools/{id}/timetable` and `POST /api/schools/{id}/timetable/pin`.
- Updated the protocol documentation to include detailed descriptions of the new timetable features and message structures.
- Enhanced the client-side implementation to support the new timetable functionalities, including lesson pinning and unpinning.
- Revised server-side logic to handle timetable operations and ensure proper integration with existing school management features.
- Added tests to validate the new timetable functionalities and ensure robustness in handling lesson data.

src/HSchool.Server/Api/SchoolEndpoints.cs

@@ -8,6 +8,7 @@ namespace HSchool.Server.Api;
 /// <summary>
 /// The main menu talks to these: list, create, delete. People list and staffing read published
 /// snapshots; the person card, hire and subject changes go through the school's mailbox.
+/// The timetable is a published snapshot; pin/unpin go through the mailbox.
 /// </summary>
 internal static class SchoolEndpoints
 {