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/dto/post_converter.go

@@ -50,6 +50,7 @@ func ConvertPostToResponse(post *model.Post, isLiked, isFavorited bool) *PostRes
 	return &PostResponse{
 		ID:              post.ID,
 		UserID:          post.UserID,
+		ChannelID:       post.ChannelID,
 		Title:           post.Title,
 		Content:         post.Content,
 		Images:          images,
@@ -90,6 +91,7 @@ func ConvertPostToDetailResponse(post *model.Post, isLiked, isFavorited bool) *P
 	return &PostDetailResponse{
 		ID:              post.ID,
 		UserID:          post.UserID,
+		ChannelID:       post.ChannelID,
 		Title:           post.Title,
 		Content:         post.Content,
 		Images:          images,