Add categories endpoint in API (#460)

* Add categories endpoint in API

* FIX eager load parent and subcategories associations

* FIX update specs to match

* Add rswag spec

* FIX openapi spec

* FIX final warns

app/views/api/v1/categories/_category.json.jbuilder

@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+
+json.id category.id
+json.name category.name
+json.classification category.classification
+json.color category.color
+json.icon category.lucide_icon
+
+# Parent information (for subcategories)
+if category.parent.present?
+  json.parent do
+    json.id category.parent.id
+    json.name category.parent.name
+  end
+else
+  json.parent nil
+end
+
+# Subcategories count (for parent categories)
+json.subcategories_count category.subcategories.size
+
+json.created_at category.created_at.iso8601
+json.updated_at category.updated_at.iso8601

app/views/api/v1/categories/index.json.jbuilder

@@ -0,0 +1,12 @@
+# frozen_string_literal: true
+
+json.categories @categories do |category|
+  json.partial! "api/v1/categories/category", category: category
+end
+
+json.pagination do
+  json.page @pagy.page
+  json.per_page @per_page
+  json.total_count @pagy.count
+  json.total_pages @pagy.pages
+end

app/views/api/v1/categories/show.json.jbuilder

@@ -0,0 +1,3 @@
+# frozen_string_literal: true
+
+json.partial! "api/v1/categories/category", category: @category