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

controllers/api/posts_helpers.go

@@ -195,6 +195,10 @@ func serializeCategoryNode(c *models.Category, byParent map[uint][]models.Catego
 	if c.Description != nil {
 		desc = *c.Description
 	}
+	var content any
+	if c.Content != nil {
+		content = *c.Content
+	}
 	var parent any
 	if c.ParentID != nil {
 		parent = *c.ParentID
@@ -208,6 +212,7 @@ func serializeCategoryNode(c *models.Category, byParent map[uint][]models.Catego
 		"name":        c.Name,
 		"slug":        c.Slug,
 		"description": desc,
+		"content":     content,
 		"parent_id":   parent,
 		"nest_depth":  depth,
 		"post_count":  postCount,