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/post_repo.go

@@ -166,7 +166,7 @@ func (r *PostRepository) Delete(id string) error {
 
 // List 分页获取帖子列表
 // includePending=true 时，仅在指定 userID 下额外返回 pending（用于作者查看自己待审核帖子）
-func (r *PostRepository) List(page, pageSize int, userID string, includePending bool) ([]*model.Post, int64, error) {
+func (r *PostRepository) List(page, pageSize int, userID string, includePending bool, channelID *string) ([]*model.Post, int64, error) {
 	var posts []*model.Post
 	var total int64
 
@@ -183,6 +183,9 @@ func (r *PostRepository) List(page, pageSize int, userID string, includePending
 	} else {
 		query = query.Where("status = ?", model.PostStatusPublished)
 	}
+	if channelID != nil && *channelID != "" {
+		query = query.Where("channel_id = ?", *channelID)
+	}
 
 	query.Count(&total)
 
@@ -849,7 +852,7 @@ func (r *PostRepository) UpdateFeatureStatus(postID string, isFeatured bool) err
 
 // GetPostsByCursor 游标分页获取帖子列表
 // includePending=true 时，仅在指定 userID 下额外返回 pending（用于作者查看自己待审核帖子）
-func (r *PostRepository) GetPostsByCursor(ctx context.Context, userID string, includePending bool, req *cursor.PageRequest) (*cursor.CursorPageResult[*model.Post], error) {
+func (r *PostRepository) GetPostsByCursor(ctx context.Context, userID string, includePending bool, channelID *string, req *cursor.PageRequest) (*cursor.CursorPageResult[*model.Post], error) {
 	db := r.getDB(ctx).WithContext(ctx)
 
 	// 构建基础查询
@@ -866,6 +869,9 @@ func (r *PostRepository) GetPostsByCursor(ctx context.Context, userID string, in
 	} else {
 		query = query.Where("status = ?", model.PostStatusPublished)
 	}
+	if channelID != nil && *channelID != "" {
+		query = query.Where("channel_id = ?", *channelID)
+	}
 
 	// 使用游标构建器
 	builder := cursor.NewBuilder(query, cursor.SortByCreatedAtDesc).