feat(channel): integrate channel functionality into post management

- Added Channel support in Post model, including ChannelID field in PostRequest and PostResponse DTOs.
- Updated PostService and PostRepository to handle channel-specific logic for creating and listing posts.
- Introduced ChannelRepository and ChannelService, along with corresponding handlers for managing channels.
- Enhanced router to include routes for channel management and public channel listing.
- Seeded default channels in the database during initialization.

internal/repository/channel_repo.go

@@ -0,0 +1,54 @@
+package repository
+
+import (
+	"carrot_bbs/internal/model"
+
+	"gorm.io/gorm"
+)
+
+// ChannelRepository 频道配置仓储
+type ChannelRepository struct {
+	db *gorm.DB
+}
+
+func NewChannelRepository(db *gorm.DB) *ChannelRepository {
+	return &ChannelRepository{db: db}
+}
+
+func (r *ChannelRepository) ListActive() ([]*model.Channel, error) {
+	var channels []*model.Channel
+	err := r.db.
+		Where("is_active = ?", true).
+		Order("sort_order ASC, created_at ASC").
+		Find(&channels).Error
+	return channels, err
+}
+
+func (r *ChannelRepository) ListAll() ([]*model.Channel, error) {
+	var channels []*model.Channel
+	err := r.db.
+		Order("sort_order ASC, created_at ASC").
+		Find(&channels).Error
+	return channels, err
+}
+
+func (r *ChannelRepository) Create(channel *model.Channel) error {
+	return r.db.Create(channel).Error
+}
+
+func (r *ChannelRepository) Update(channel *model.Channel) error {
+	return r.db.Save(channel).Error
+}
+
+func (r *ChannelRepository) GetByID(id string) (*model.Channel, error) {
+	var channel model.Channel
+	if err := r.db.First(&channel, "id = ?", id).Error; err != nil {
+		return nil, err
+	}
+	return &channel, nil
+}
+
+func (r *ChannelRepository) Delete(id string) error {
+	return r.db.Delete(&model.Channel{}, "id = ?", id).Error
+}
+