diff --git a/docs/superpowers/specs/2026-03-18-categories-design.md b/docs/superpowers/specs/2026-03-18-categories-design.md
index eb5a004..9c10abb 100644
--- a/docs/superpowers/specs/2026-03-18-categories-design.md
+++ b/docs/superpowers/specs/2026-03-18-categories-design.md
@@ -17,6 +17,7 @@ CREATE TABLE categories (
     slug        TEXT    NOT NULL,
     description TEXT    NOT NULL DEFAULT '',
     created_at  INTEGER NOT NULL,
+    updated_at  INTEGER NOT NULL,
     FOREIGN KEY (site_id) REFERENCES sites (id) ON DELETE CASCADE
 );
 CREATE INDEX idx_categories_site ON categories (site_id);
@@ -44,11 +45,13 @@ type Category struct {
     Slug        string    `json:"slug"`
     Description string    `json:"description"`
     CreatedAt   time.Time `json:"created_at"`
+    UpdatedAt   time.Time `json:"updated_at"`
 }
 ```
 
 - `slug` is auto-generated from `name` (e.g. "Go Programming" -> `go-programming`), editable by the user.
 - `description` is Markdown, rendered on the category archive page. Defaults to empty string.
+- DB provider must use the existing `timeToInt()`/`time.Unix()` helpers for timestamp conversion, consistent with how posts are handled.
 
 ## Admin UI
 
@@ -66,7 +69,7 @@ Templates: `views/categories/index.html`, `views/categories/edit.html`.
 
 ### Post Edit Form Changes
 
-- A multi-select checkbox list of all available categories, displayed in a **right sidebar** alongside the main title/body editing area on the left.
+- A multi-select checkbox list of all available categories (sorted alphabetically by name), displayed in a **right sidebar** alongside the main title/body editing area on the left.
 - Selected category IDs sent with the form submission.
 - `CreatePostParams` gains `CategoryIDs []int64`.
 
@@ -114,8 +117,8 @@ New file: `sql/queries/categories.sql`
 - `SelectCategory` — single category by ID
 - `SelectCategoryByGUID` — single category by GUID
 - `SelectCategoriesOfPost` — categories for a given post (via join table)
-- `SelectPostsOfCategory` — published, non-deleted posts in a category, ordered by `published_at` desc
-- `CountPostsOfCategory` — count of published posts per category
+- `SelectPostsOfCategory` — published, non-deleted posts in a category (`state = 0 AND deleted_at = 0`), ordered by `published_at` desc
+- `CountPostsOfCategory` — count of published posts per category (same `state = 0 AND deleted_at = 0` filter)
 - `InsertCategory` / `UpdateCategory` / `DeleteCategory` — CRUD
 - `InsertPostCategory` / `DeletePostCategory` — manage the join table
 - `DeletePostCategoriesByPost` — clear all categories for a post (delete-then-reinsert on save)
@@ -128,25 +131,28 @@ New file: `sql/queries/categories.sql`
 
 - `ListCategories(ctx) ([]Category, error)` — all categories for the current site (from context)
 - `GetCategory(ctx, id) (*Category, error)`
-- `CreateCategory(ctx, params) (*Category, error)` — auto-generates slug from name
-- `UpdateCategory(ctx, params) (*Category, error)`
+- `CreateCategory(ctx, params) (*Category, error)` — auto-generates slug from name. If the slug collides with an existing one for the same site, return a validation error.
+- `UpdateCategory(ctx, params) (*Category, error)` — same slug collision check on update.
 - `DeleteCategory(ctx, id) error` — deletes category and post associations, queues site rebuild
 
+All mutation methods verify site ownership (same pattern as post service authorization checks).
+
 ### Changes to `services/posts`
 
-- `UpdatePost` — after saving the post, deletes existing `post_categories` rows and re-inserts for the selected category IDs
+- `UpdatePost` — after saving the post, deletes existing `post_categories` rows and re-inserts for the selected category IDs. The post save and category reassignment must run within a single database transaction to ensure atomicity.
 - `GetPost` / `ListPosts` — loads each post's categories for admin display
 
 ### Changes to Publishing Pipeline
 
 - `pubmodel.Site` gains new fields:
-  - Category list (with post counts and description excerpts for the index page)
-  - A function to iterate published posts by category
+  - `Categories []CategoryWithCount` — category list with post counts and description excerpts for the index page
+  - `PostIterByCategory func(ctx context.Context, categoryID int64) iter.Seq[models.Maybe[*models.Post]]` — iterator for posts in a specific category
 - `sitebuilder.Builder.BuildSite` gains additional goroutines for:
   - Rendering the category index page
   - Rendering each category archive page
   - Rendering per-category feeds
-- New templates: `tmplNameCategoryList`, `tmplNameCategorySingle`
+- New templates: `tmplNameCategoryList`, `tmplNameCategorySingle` (must be added to the `ParseFS` call in `sitebuilder.New()`)
+- `postSingleData` gains a `Categories []Category` field so post templates can render category links
 
 ### Rebuild Triggers
 
@@ -155,3 +161,9 @@ Saving or deleting a category queues a site rebuild, same as post state changes.
 ## DB Provider
 
 `providers/db/` gains wrapper methods for all new sqlc queries, following the same pattern as existing post methods (e.g. `SaveCategory`, `SelectCategoriesOfPost`, etc.).
+
+## Design Decisions
+
+- **Hard delete for categories** — unlike posts which use soft-delete, categories are hard-deleted. They are simpler entities and don't need a trash/restore workflow.
+- **No sort_order column** — categories are sorted alphabetically by name. Manual ordering can be added later if needed.
+- **Existing microblog-crosspost feed** — kept as-is. Per-category feeds are a separate, additive feature.