Carry a store's configuration in the registry record

A store's `config.json` lived in a repository the desktop client fetched over HTTP,
which made a store's behaviour depend on a second thing existing and staying
reachable. The registry already answers what a store *is*; it now answers how it
behaves too, in the same shape that file had, so this record is the one source of
truth and a store can be configured from the admin alone.

`storeRepositoryUrl` stays, demoted to a pointer for a person — where the store's
own repository is, when it has one. Clients released before this field still fetch a
`config.json` from it, so nothing has to move at once.

The column is nullable because a store that configures nothing is still a store: the
client falls back to the engine's built-in defaults, which need only a name and a
catalog. The admin edits it as JSON text through a pair of accessors, so the column
holds real JSON and invalid input comes back with the text kept and a message rather
than a 500.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

apps/api/db/migrate/20260819000001_carry_the_store_config_in_the_registry.rb

@@ -0,0 +1,16 @@
+class CarryTheStoreConfigInTheRegistry < ActiveRecord::Migration[8.1]
+  # The store's own configuration moves into this record.
+  #
+  # It used to live as a `config.json` in a repository the client fetched over HTTP,
+  # which made a store's behaviour depend on a second thing existing and staying
+  # reachable. The registry already answers what a store *is*; carrying how it behaves
+  # in the same record makes this the one source of truth, and lets a store exist with
+  # no repository at all — which is the ordinary case now that the store engine ships
+  # inside the client.
+  #
+  # Nullable, because a store that configures nothing is still a store: the client
+  # falls back to the engine's built-in defaults, which need only a name and a catalog.
+  def change
+    add_column :stores, :config, :json
+  end
+end

apps/api/db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[8.1].define(version: 2026_08_18_120000) do
+ActiveRecord::Schema[8.1].define(version: 2026_08_19_000001) do
   create_table "admin_users", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
     t.datetime "created_at", null: false
     t.datetime "deleted_at", precision: 3
@@ -283,6 +283,7 @@ ActiveRecord::Schema[8.1].define(version: 2026_08_18_120000) do
 
   create_table "stores", id: { type: :bigint, unsigned: true }, charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
     t.string "catalog_url", null: false
+    t.json "config"
     t.datetime "created_at", precision: 3, null: false
     t.datetime "deleted_at", precision: 3
     t.string "name", null: false