#217 docs(sessions): add endpoint to mark chat as read

• Introduced a new POST endpoint /api/sessions/{sessionId}/chats/{chatId}/read to mark a session chat as read. This endpoint updates the lastReadAt timestamp for the specified chat and ensures idempotency.
• Updated the OpenAPI specification with detailed request parameters and response schemas, including success and error responses.
• Enhanced the documentation to include the new endpoint in the API reference, improving clarity for users managing session chats.

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Summary by cubic

Documented a new POST endpoint to mark a session chat as read and updated the OpenAPI spec and API reference. This lets clients clear unread state (hasUnread becomes false) for a chat until new assistant activity arrives.

• New Features
  • Added POST /api/sessions/{sessionId}/chats/{chatId}/read to upsert a read receipt and set lastReadAt. Idempotent.
  • OpenAPI: path params sessionId and chatId. 200 returns MarkChatReadResponse { success: true }. Added 401, 403, 404, 500 errors.
  • Docs: new API page and nav entry under Sessions.

Summary by CodeRabbit

• New Features

  • Added a new POST /api/sessions/{sessionId}/chats/{chatId}/read endpoint with idempotent behavior and comprehensive error handling.

• Documentation

  • Added documentation page for the new endpoint.
  • Updated API reference navigation to include the new resource.