feat(posts): add post reference/internal linking functionality

Add support for referencing other posts within messages through a new post_ref segment type. Includes new PostReference model to track relationships, PostRefService for processing references, and new endpoints for post suggestions, related posts, and reference click tracking.

internal/model/post_reference.go

@@ -0,0 +1,40 @@
+package model
+
+import (
+	"time"
+
+	"github.com/google/uuid"
+	"gorm.io/gorm"
+)
+
+type PostReferenceStatus string
+
+const (
+	PostRefActive PostReferenceStatus = "active"
+	PostRefDead   PostReferenceStatus = "dead"
+	PostRefHidden PostReferenceStatus = "hidden"
+)
+
+type PostReference struct {
+	ID           string               `json:"id" gorm:"type:varchar(36);primaryKey"`
+	SourcePostID string               `json:"source_post_id" gorm:"type:varchar(36);not null;uniqueIndex:uq_source_target;index:idx_source_post"`
+	TargetPostID string               `json:"target_post_id" gorm:"type:varchar(36);not null;uniqueIndex:uq_source_target;index:idx_target_post;index:idx_target_status"`
+	Status       PostReferenceStatus  `json:"status" gorm:"type:varchar(20);default:active;index:idx_target_status"`
+	ClickCount   int                  `json:"click_count" gorm:"default:0"`
+	CreatedAt    time.Time            `json:"created_at" gorm:"autoCreateTime"`
+	UpdatedAt    time.Time            `json:"updated_at" gorm:"autoUpdateTime:false"`
+
+	SourcePost *Post `json:"source_post,omitempty" gorm:"foreignKey:SourcePostID"`
+	TargetPost *Post `json:"target_post,omitempty" gorm:"foreignKey:TargetPostID"`
+}
+
+func (pr *PostReference) BeforeCreate(tx *gorm.DB) error {
+	if pr.ID == "" {
+		pr.ID = uuid.New().String()
+	}
+	return nil
+}
+
+func (PostReference) TableName() string {
+	return "post_references"
+}