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/repository/grade_repo.go

@@ -9,11 +9,12 @@ import (
 )
 
 type GradeRepository interface {
-	ListByUserAndSemester(userID, semester string) ([]*model.Grade, error)
-	DeleteByUserAndSemester(ctx context.Context, userID, semester string) error
+	ListByUser(userID string) ([]*model.Grade, error)
+	DeleteByUser(ctx context.Context, userID string) error
 	BatchCreate(ctx context.Context, grades []*model.Grade) error
-	GetGpaSummary(userID, semester string) (*model.GpaSummary, error)
+	GetLatestGpaSummary(userID string) (*model.GpaSummary, error)
 	SaveGpaSummary(ctx context.Context, summary *model.GpaSummary) error
+	DeleteGpaSummaryByUser(ctx context.Context, userID string) error
 }
 
 type gradeRepository struct {
@@ -24,17 +25,17 @@ func NewGradeRepository(db *gorm.DB) GradeRepository {
 	return &gradeRepository{db: db}
 }
 
-func (r *gradeRepository) ListByUserAndSemester(userID, semester string) ([]*model.Grade, error) {
+func (r *gradeRepository) ListByUser(userID string) ([]*model.Grade, error) {
 	var grades []*model.Grade
-	err := r.db.Where("user_id = ? AND term = ?", userID, semester).
-		Order("created_at ASC").
+	err := r.db.Where("user_id = ?", userID).
+		Order("term DESC, created_at ASC").
 		Find(&grades).Error
 	return grades, err
 }
 
-func (r *gradeRepository) DeleteByUserAndSemester(ctx context.Context, userID, semester string) error {
+func (r *gradeRepository) DeleteByUser(ctx context.Context, userID string) error {
 	return r.db.WithContext(ctx).
-		Where("user_id = ? AND term = ?", userID, semester).
+		Where("user_id = ?", userID).
 		Delete(&model.Grade{}).Error
 }
 
@@ -42,9 +43,11 @@ func (r *gradeRepository) BatchCreate(ctx context.Context, grades []*model.Grade
 	return r.db.WithContext(ctx).CreateInBatches(grades, 100).Error
 }
 
-func (r *gradeRepository) GetGpaSummary(userID, semester string) (*model.GpaSummary, error) {
+func (r *gradeRepository) GetLatestGpaSummary(userID string) (*model.GpaSummary, error) {
 	var summary model.GpaSummary
-	err := r.db.Where("user_id = ? AND semester = ?", userID, semester).First(&summary).Error
+	err := r.db.Where("user_id = ?", userID).
+		Order("updated_at DESC").
+		First(&summary).Error
 	if err != nil {
 		return nil, err
 	}
@@ -54,3 +57,9 @@ func (r *gradeRepository) GetGpaSummary(userID, semester string) (*model.GpaSumm
 func (r *gradeRepository) SaveGpaSummary(ctx context.Context, summary *model.GpaSummary) error {
 	return r.db.WithContext(ctx).Save(summary).Error
 }
+
+func (r *gradeRepository) DeleteGpaSummaryByUser(ctx context.Context, userID string) error {
+	return r.db.WithContext(ctx).
+		Where("user_id = ?", userID).
+		Delete(&model.GpaSummary{}).Error
+}