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/model/call.go

@@ -43,7 +43,9 @@ type CallSession struct {
 	GroupID        *string     `gorm:"index" json:"group_id,omitempty"`                                       // 群组ID（群聊时使用）
 	CallerID       string      `gorm:"column:caller_id;type:varchar(50);index;not null" json:"caller_id"`     // 发起者ID
 	CallType       CallType    `gorm:"type:varchar(20);not null" json:"call_type"`                            // 通话类型
+	MediaType      string      `gorm:"type:varchar(20);default:'voice'" json:"media_type"`                    // 媒体类型: voice/video
 	Status         CallStatus  `gorm:"type:varchar(20);default:'calling'" json:"status"`                      // 通话状态
+	EndedBy        *string     `gorm:"type:varchar(50)" json:"ended_by,omitempty"`                            // 结束通话的用户ID
 	StartedAt      *time.Time  `json:"started_at,omitempty"`                                                  // 接通时间
 	EndedAt        *time.Time  `json:"ended_at,omitempty"`                                                    // 结束时间
 	Duration       int64       `gorm:"default:0" json:"duration"`                                             // 通话时长（秒）