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/ws_handler.go

@@ -377,6 +377,8 @@ func (h *WSHandler) handleMessage(client *ws.Client, msg *ws.Message) {
 	// 通话信令
 	case "call_invite":
 		h.handleCallInvite(ctx, client, msg.Payload)
+	case "call_group_invite":
+		h.handleCallGroupInvite(ctx, client, msg.Payload)
 	case "call_answer":
 		h.handleCallAnswer(ctx, client, msg.Payload)
 	case "call_reject":
@@ -389,6 +391,8 @@ func (h *WSHandler) handleMessage(client *ws.Client, msg *ws.Message) {
 		h.handleCallEnd(ctx, client, msg.Payload)
 	case "call_mute":
 		h.handleCallMute(ctx, client, msg.Payload)
+	case "call_participant_join":
+		h.handleCallParticipantJoin(ctx, client, msg.Payload)
 	default:
 		h.publisher.SendError(client, "unknown_type", fmt.Sprintf("unknown message type: %s", msg.Type))
 	}
@@ -648,6 +652,61 @@ func (h *WSHandler) handleCallInvite(ctx context.Context, client *ws.Client, pay
 	}
 }
 
+// handleCallGroupInvite 处理群组通话邀请
+func (h *WSHandler) handleCallGroupInvite(ctx context.Context, client *ws.Client, payload json.RawMessage) {
+	if !h.isVerified(ctx, client) {
+		return
+	}
+
+	var req struct {
+		GroupID        string `json:"group_id"`
+		ConversationID string `json:"conversation_id"`
+		MediaType      string `json:"call_type"`
+	}
+	if err := json.Unmarshal(payload, &req); err != nil {
+		h.publisher.SendError(client, "parse_error", "invalid call_group_invite format")
+		return
+	}
+	if req.GroupID == "" || req.ConversationID == "" {
+		h.publisher.SendError(client, "invalid_params", "group_id and conversation_id are required")
+		return
+	}
+
+	mediaType := req.MediaType
+	if mediaType != "video" {
+		mediaType = "voice"
+	}
+
+	call, err := h.callService.GroupInvite(ctx, client.UserID, req.GroupID, req.ConversationID, mediaType)
+	if err != nil {
+		zap.L().Warn("Failed to invite group call",
+			zap.String("caller_id", client.UserID),
+			zap.String("group_id", req.GroupID),
+			zap.Error(err),
+		)
+		h.publisher.SendError(client, "call_group_invite_failed", err.Error())
+		return
+	}
+
+	resp := ws.ResponseMessage{
+		EventID: h.publisher.NextID(),
+		Type:    "call_invited",
+		TS:      time.Now().UnixMilli(),
+		Payload: map[string]any{
+			"call_id":         call.ID,
+			"conversation_id": req.ConversationID,
+			"group_id":        req.GroupID,
+			"call_type":       "group",
+			"lifetime":        service.CallLifetimeMs,
+		},
+	}
+	data, _ := json.Marshal(resp)
+	select {
+	case <-client.Quit:
+	case client.Send <- data:
+	}
+}
+
 // handleCallAnswer 处理接听
 func (h *WSHandler) handleCallAnswer(ctx context.Context, client *ws.Client, payload json.RawMessage) {
 	var req struct {
@@ -778,3 +837,18 @@ func (h *WSHandler) handleCallMute(ctx context.Context, client *ws.Client, paylo
 		h.publisher.SendError(client, "call_mute_failed", err.Error())
 	}
 }
+
+// handleCallParticipantJoin 处理群组通话参与者加入
+func (h *WSHandler) handleCallParticipantJoin(ctx context.Context, client *ws.Client, payload json.RawMessage) {
+	var req struct {
+		CallID string `json:"call_id"`
+	}
+	if err := json.Unmarshal(payload, &req); err != nil || req.CallID == "" {
+		h.publisher.SendError(client, "invalid_params", "call_id is required")
+		return
+	}
+
+	if err := h.callService.ParticipantJoin(ctx, req.CallID, client.UserID); err != nil {
+		h.publisher.SendError(client, "call_participant_join_failed", err.Error())
+	}
+}