feat(api): add message segments support for posts and comments

Add rich message segment support enabling structured content (text, images, votes, mentions) in posts and comments. Includes segments field in models, DTOs, handlers, and services. Also adds @mention notification processing and migrates vote data to embedded segments.

internal/model/post.go

@@ -20,11 +20,12 @@ const (
 
 // Post 帖子实体
 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"`
-	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"`
+	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"`
+	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"`
+	Segments  MessageSegments `json:"segments,omitempty" gorm:"type:text"`
 
 	// 关联
 	// User 需要参与缓存序列化；否则列表命中缓存后会丢失作者信息，前端退化为“匿名用户”