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/livekit_handler.go

@@ -127,10 +127,21 @@ func (h *LiveKitHandler) HandleWebhook(c *gin.Context) {
 	case "room_finished":
 		h.handleRoomFinished(c.Request.Context(), event)
 	case "participant_left":
+		identity := event.Participant.GetIdentity()
+		roomName := event.Room.GetName()
 		h.logger.Info("participant left room",
-			zap.String("room", event.Room.GetName()),
-			zap.String("identity", event.Participant.GetIdentity()),
+			zap.String("room", roomName),
+			zap.String("identity", identity),
 		)
+		if identity != "" && roomName != "" {
+			if err := h.callService.ParticipantLeave(c.Request.Context(), roomName, identity, "left_livekit"); err != nil {
+				h.logger.Warn("failed to handle participant_left",
+					zap.String("room", roomName),
+					zap.String("identity", identity),
+					zap.Error(err),
+				)
+			}
+		}
 	}
 
 	c.JSON(http.StatusOK, gin.H{"status": "ok"})