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/service/admin_post_service.go

@@ -322,7 +322,7 @@ func convertPostToAdminDetailResponse(post *model.Post) *dto.AdminPostDetailResp
 	return &dto.AdminPostDetailResponse{
 		ID:              post.ID,
 		UserID:          post.UserID,
-		CommunityID:     post.CommunityID,
+		ChannelID:       post.ChannelID,
 		Title:           post.Title,
 		Content:         post.Content,
 		Images:          images,