refactor(layout): consolidate backend codebase into backend/ package and clean root directory

- Moved cmd/, core/, plugins/, pkg/, downstream/, and main.go into backend/ directory
- Batch updated all Go source files to import github.com/Rain-kl/Wavelet/backend/...
- Updated Makefile, scripts/swagger.sh, architecture guards, and platform skills
- Passed all quality gates (100% tests, 0 lint issues, clean build)

backend/plugins/domain/message_gateway/logics.go

@@ -0,0 +1,155 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package message_gateway
+
+import (
+	"context"
+	"errors"
+	"strconv"
+	"strings"
+	"time"
+
+	"gorm.io/gorm"
+)
+
+// BindRequest is the user bind body.
+type BindRequest struct {
+	ChannelID string `json:"channel_id"`
+	Code      string `json:"code"`
+}
+
+// BindingDTO is a user-facing binding row.
+type BindingDTO struct {
+	ID             uint64    `json:"id,string"`
+	UserID         uint64    `json:"user_id,string"`
+	ChannelID      uint64    `json:"channel_id,string"`
+	ChannelName    string    `json:"channel_name"`
+	ChannelType    string    `json:"channel_type"`
+	PlatformUserID string    `json:"platform_user_id"`
+	CreatedAt      time.Time `json:"created_at"`
+}
+
+func bindChannel(ctx context.Context, userID uint64, req BindRequest) (BindingDTO, error) {
+	channelID, err := strconv.ParseUint(strings.TrimSpace(req.ChannelID), 10, 64)
+	if err != nil || channelID == 0 {
+		return BindingDTO{}, errChannelIDRequired
+	}
+	code := NormalizeCode(req.Code)
+	if code == "" {
+		return BindingDTO{}, errCodeInvalid
+	}
+	pairing, err := GetPairingCode(ctx, code)
+	if err != nil {
+		if errors.Is(err, gorm.ErrRecordNotFound) {
+			return BindingDTO{}, errCodeInvalid
+		}
+		return BindingDTO{}, err
+	}
+	if !pairing.ExpiresAt.After(time.Now()) {
+		return BindingDTO{}, errCodeInvalid
+	}
+	if pairing.ChannelID != channelID {
+		return BindingDTO{}, errChannelMismatch
+	}
+	ch, err := GetMessageChannel(ctx, channelID)
+	if err != nil {
+		if errors.Is(err, gorm.ErrRecordNotFound) {
+			return BindingDTO{}, errCodeInvalid
+		}
+		return BindingDTO{}, err
+	}
+	if !ch.Enabled {
+		return BindingDTO{}, errChannelDisabled
+	}
+
+	existing, err := GetBindingByChannelPlatform(ctx, channelID, pairing.PlatformUserID)
+	if err != nil && !errors.Is(err, gorm.ErrRecordNotFound) {
+		return BindingDTO{}, err
+	}
+	if err == nil && existing != nil {
+		if existing.UserID != userID {
+			return BindingDTO{}, errPlatformAlreadyBound
+		}
+		_ = DeletePairingCode(ctx, pairing.Code)
+		return toBindingDTO(existing, ch), nil
+	}
+
+	row := &MessageBinding{
+		UserID:         userID,
+		ChannelID:      channelID,
+		PlatformUserID: pairing.PlatformUserID,
+	}
+	if err := CreateMessageBinding(ctx, row); err != nil {
+		return BindingDTO{}, err
+	}
+	if err := DeletePairingCode(ctx, pairing.Code); err != nil {
+		return BindingDTO{}, err
+	}
+	return toBindingDTO(row, ch), nil
+}
+
+// PublicChannelDTO is an enabled channel a user can bind to.
+type PublicChannelDTO struct {
+	ID   uint64 `json:"id,string"`
+	Name string `json:"name"`
+	Type string `json:"type"`
+}
+
+func listEnabledPublicChannels(ctx context.Context) ([]PublicChannelDTO, error) {
+	rows, err := ListEnabledMessageChannels(ctx)
+	if err != nil {
+		return nil, err
+	}
+	out := make([]PublicChannelDTO, 0, len(rows))
+	for _, row := range rows {
+		out = append(out, PublicChannelDTO{ID: row.ID, Name: row.Name, Type: row.Type})
+	}
+	return out, nil
+}
+
+func listUserBindings(ctx context.Context, userID uint64) ([]BindingDTO, error) {
+	rows, err := ListBindingsByUser(ctx, userID)
+	if err != nil {
+		return nil, err
+	}
+	out := make([]BindingDTO, 0, len(rows))
+	for i := range rows {
+		ch, err := GetMessageChannel(ctx, rows[i].ChannelID)
+		if err != nil {
+			out = append(out, toBindingDTO(&rows[i], nil))
+			continue
+		}
+		out = append(out, toBindingDTO(&rows[i], ch))
+	}
+	return out, nil
+}
+
+func unbindChannel(ctx context.Context, userID, bindingID uint64) error {
+	row, err := GetMessageBinding(ctx, bindingID)
+	if err != nil {
+		if errors.Is(err, gorm.ErrRecordNotFound) {
+			return errBindingNotFound
+		}
+		return err
+	}
+	if row.UserID != userID {
+		return errBindingForbidden
+	}
+	return DeleteMessageBinding(ctx, bindingID)
+}
+
+func toBindingDTO(row *MessageBinding, ch *MessageChannel) BindingDTO {
+	dto := BindingDTO{
+		ID:             row.ID,
+		UserID:         row.UserID,
+		ChannelID:      row.ChannelID,
+		PlatformUserID: row.PlatformUserID,
+		CreatedAt:      row.CreatedAt,
+	}
+	if ch != nil {
+		dto.ChannelName = ch.Name
+		dto.ChannelType = ch.Type
+	}
+	return dto
+}