refactor: 统一 BaseModel 嵌入，修复 primaryKey 大小写不一致

变更模型:
- Post: 嵌入 BaseModel，移除手动 ID/CreatedAt/UpdatedAt/DeletedAt
- Comment: 嵌入 BaseModel，保留 is_deleted 自定义软删除
- Announcement: 嵌入 BaseModel
- Category: 嵌入 BaseModel
- Folder + FolderItem: 嵌入 BaseModel
- CommunityFund: primaryKey → primarykey (统一小写)

internal/model/category.go

@@ -1,17 +1,14 @@
 package model
 
-import "time"
-
 // Category 文章分类（两级树形结构）
 type Category struct {
-	ID          uint      `gorm:"primarykey" json:"id"`
-	Name        string    `gorm:"type:varchar(50);not null" json:"name"`
-	Slug        string    `gorm:"type:varchar(50);not null;uniqueIndex" json:"slug"`
-	Description string    `gorm:"type:varchar(200);default:''" json:"description,omitempty"`
-	ParentID    *uint     `gorm:"index" json:"parent_id,omitempty"`
-	Sort        int       `gorm:"default:0" json:"sort"`
-	CreatedAt   time.Time `json:"created_at"`
-	UpdatedAt   time.Time `json:"updated_at"`
+	BaseModel
+
+	Name        string `gorm:"type:varchar(50);not null" json:"name"`
+	Slug        string `gorm:"type:varchar(50);not null;uniqueIndex" json:"slug"`
+	Description string `gorm:"type:varchar(200);default:''" json:"description,omitempty"`
+	ParentID    *uint  `gorm:"index" json:"parent_id,omitempty"`
+	Sort        int    `gorm:"default:0" json:"sort"`
 
 	// 非 DB 字段（树形填充）
 	Children []Category `gorm:"-" json:"children,omitempty"`