feat(call): implement group calling and enhance call management

Introduces group calling capabilities, including group invites and participant
lifecycle management. Enhances the existing call system with support for
media types (voice/video) and improved participant tracking.

- Add `GroupInvite` and `ParticipantJoin/Leave` to `CallService`.
- Implement WebSocket handlers for `call_group_invite` and
  `call_participant_join`.
- Update `CallSession` model to support group IDs, media types, and
  tracking who ended the call.
- Integrate `GroupService` into `CallService` via dependency injection.
- Add error constants for group call limitations and participant capacity.
- Update LiveKit webhook handling to process participant leave events.

Refactor grade synchronization to remove semester-based filtering in favor
of a more flexible user-based approach and improve GPA summary updates.

internal/errors/app_errors.go

@@ -108,6 +108,8 @@ var (
 	ErrInvalidCallState    = &AppError{Code: "INVALID_CALL_STATE", Message: "通话状态无效"}
 	ErrCalleeOffline       = &AppError{Code: "CALLEE_OFFLINE", Message: "对方不在线"}
 	ErrCallAlreadyAnswered = &AppError{Code: "CALL_ALREADY_ANSWERED", Message: "通话已在其他设备应答"}
+	ErrGroupCallNotSupported = &AppError{Code: "GROUP_CALL_NOT_SUPPORTED", Message: "群组通话不支持"}
+	ErrMaxParticipantsReached = &AppError{Code: "MAX_PARTICIPANTS_REACHED", Message: "通话人数已达上限"}
 
 	// 消息相关错误
 	ErrConversationNotFound   = &AppError{Code: "CONVERSATION_NOT_FOUND", Message: "会话不存在或无权限"}