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/model/post.go

@@ -22,7 +22,7 @@ const (
 type Post struct {
 	ID          string `json:"id" gorm:"type:varchar(36);primaryKey"`
 	UserID      string `json:"user_id" gorm:"type:varchar(36);index;index:idx_posts_user_status_created,priority:1;not null"`
-	CommunityID string `json:"community_id" gorm:"type:varchar(36);index"`
+	ChannelID *string `json:"channel_id,omitempty" gorm:"type:varchar(36);index"`
 	Title       string `json:"title" gorm:"type:varchar(200);not null"`
 	Content     string `json:"content" gorm:"type:text;not null"`