feat(api): serve the category content field on GET /_journal/api/v1/categories

- add the nullable TEXT column golem15_journal_categories.content (migration 202610090001)
- map it as models.Category.Content and emit category content on every categories row
- document the categories row shape and the stored-as-is HTML semantics

models/category.go

@@ -9,6 +9,7 @@ type Category struct {
 	Slug        string     `gorm:"column:slug"`
 	Code        *string    `gorm:"column:code"`
 	Description *string    `gorm:"column:description"`
+	Content     *string    `gorm:"column:content"` // HTML body (D-21): stored and served as is, not translatable
 	ParentID    *uint      `gorm:"column:parent_id"`
 	NestLeft    *int       `gorm:"column:nest_left"`
 	NestRight   *int       `gorm:"column:nest_right"`