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/dto/post_converter.go

@@ -1,9 +1,9 @@
-package dto
+package dto
 
 import (
-	"with_you/internal/model"
 	"context"
 	"encoding/json"
+	"with_you/internal/model"
 )
 
 // ==================== Post 转换 ====================
@@ -67,6 +67,20 @@ func PostChannelBriefForPost(post *model.Post, channelByID map[string]*model.Cha
 	return postChannelBrief(post, channelByID)
 }
 
+// SegmentsOrDefault 如果 segments 为空但 content 非空，
+// 返回降级的单 text segment；否则原样返回。
+func SegmentsOrDefault(segments model.MessageSegments, content string) model.MessageSegments {
+	if len(segments) > 0 {
+		return segments
+	}
+	if content != "" {
+		return model.MessageSegments{
+			{Type: string(SegmentTypeText), Data: map[string]any{"text": content}},
+		}
+	}
+	return nil
+}
+
 // ConvertPostToResponse 将Post转换为PostResponse（列表用）；channelByID 可为 nil
 func ConvertPostToResponse(post *model.Post, channelByID map[string]*model.Channel, isLiked, isFavorited bool) *PostResponse {
 	if post == nil {
@@ -89,6 +103,7 @@ func ConvertPostToResponse(post *model.Post, channelByID map[string]*model.Chann
 		ChannelID:       post.ChannelID,
 		Title:           post.Title,
 		Content:         post.Content,
+		Segments:        SegmentsOrDefault(post.Segments, post.Content),
 		Images:          images,
 		Status:          string(post.Status),
 		LikesCount:      post.LikesCount,
@@ -131,6 +146,7 @@ func ConvertPostToDetailResponse(post *model.Post, channelByID map[string]*model
 		ChannelID:       post.ChannelID,
 		Title:           post.Title,
 		Content:         post.Content,
+		Segments:        SegmentsOrDefault(post.Segments, post.Content),
 		Images:          images,
 		Status:          string(post.Status),
 		LikesCount:      post.LikesCount,
@@ -227,6 +243,7 @@ func ConvertCommentToResponse(comment *model.Comment, isLiked bool) *CommentResp
 		ParentID:     comment.ParentID,
 		RootID:       comment.RootID,
 		Content:      comment.Content,
+		Segments:     SegmentsOrDefault(comment.Segments, comment.Content),
 		Images:       images,
 		LikesCount:   comment.LikesCount,
 		RepliesCount: comment.RepliesCount,
@@ -303,6 +320,7 @@ func ConvertCommentToResponseWithUser(comment *model.Comment, userID string, che
 		ParentID:     comment.ParentID,
 		RootID:       comment.RootID,
 		Content:      comment.Content,
+		Segments:     SegmentsOrDefault(comment.Segments, comment.Content),
 		Images:       images,
 		LikesCount:   comment.LikesCount,
 		RepliesCount: comment.RepliesCount,