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/handler/grade_handler.go

@@ -62,19 +62,14 @@ func (h *GradeHandler) ListGrades(c *gin.Context) {
 		response.Unauthorized(c, "")
 		return
 	}
-	semester := c.Query("semester")
-	if semester == "" {
-		response.BadRequest(c, "semester is required")
-		return
-	}
 
-	grades, err := h.gradeRepo.ListByUserAndSemester(userID, semester)
+	grades, err := h.gradeRepo.ListByUser(userID)
 	if err != nil {
 		response.HandleError(c, err, "failed to list grades")
 		return
 	}
 
-	gpaSummary, _ := h.gradeRepo.GetGpaSummary(userID, semester)
+	gpaSummary, _ := h.gradeRepo.GetLatestGpaSummary(userID)
 
 	response.Success(c, gin.H{
 		"grades":      grades,