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/announcement.go

@@ -4,14 +4,13 @@ import "time"
 
 // Announcement 公告
 type Announcement struct {
-	ID        uint       `gorm:"primarykey" json:"id"`
-	Title     string     `gorm:"type:varchar(200);not null" json:"title"`
-	Content   string     `gorm:"type:text;not null" json:"content"`
-	Type      string     `gorm:"type:varchar(20);default:info" json:"type"` // info / warning / success / error
-	IsActive  bool       `gorm:"default:true" json:"is_active"`
-	StartAt   *time.Time `json:"start_at,omitempty"`
-	EndAt     *time.Time `json:"end_at,omitempty"`
-	Sort      int        `gorm:"default:0" json:"sort"`
-	CreatedAt time.Time  `json:"created_at"`
-	UpdatedAt time.Time  `json:"updated_at"`
+	BaseModel
+
+	Title    string     `gorm:"type:varchar(200);not null" json:"title"`
+	Content  string     `gorm:"type:text;not null" json:"content"`
+	Type     string     `gorm:"type:varchar(20);default:info" json:"type"` // info / warning / success / error
+	IsActive bool       `gorm:"default:true" json:"is_active"`
+	StartAt  *time.Time `json:"start_at,omitempty"`
+	EndAt    *time.Time `json:"end_at,omitempty"`
+	Sort     int        `gorm:"default:0" json:"sort"`
 }