A store registry record needs no repository

The client only ever took identity from a store repository — a slug, a name, a catalog —
and the store engine's own defaults cover everything else: the host-to-asset mapping, the
install modes, the platforms, the behaviour. So `store_repository_url` is now optional:
nullable in the schema, no presence validation, the format check only when a value is
given, and the serializer answers null rather than an empty string, because the client
branches on its absence.

Adding a store is therefore a row with two fields filled in. Given a repository the
client still reads its config.json, and that file remains the authority on how the store
behaves — the admin form and the endpoint's documentation say so.

The frontend's /stores page gains a section of its own for the graphical client on the
desktop tab: what it does, that it sets the store up itself, that it is the way in on
Windows where `curl … | sh` does not exist, and links to the releases, the repository and
the documentation — now under stores/warp-engine-client, which is where that repository
lives after the rename.

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

apps/api/db/migrate/20260818120000_allow_stores_without_a_repository.rb

@@ -0,0 +1,10 @@
+class AllowStoresWithoutARepository < ActiveRecord::Migration[8.1]
+  # A store repository is now optional. The client only ever needed identity from
+  # it — a name, a catalog and a slug — and the engine's own defaults cover
+  # everything else, so a record with a catalog URL is a complete store. A
+  # repository is still honoured when there is one: it stays the authority on how
+  # that store behaves.
+  def change
+    change_column_null :stores, :store_repository_url, true
+  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_000001) do
+ActiveRecord::Schema[8.1].define(version: 2026_08_18_120000) 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
@@ -286,7 +286,7 @@ ActiveRecord::Schema[8.1].define(version: 2026_08_18_000001) do
     t.datetime "created_at", precision: 3, null: false
     t.datetime "deleted_at", precision: 3
     t.string "name", null: false
-    t.string "store_repository_url", null: false
+    t.string "store_repository_url"
     t.datetime "updated_at", precision: 3, null: false
     t.index ["deleted_at"], name: "idx_stores_deleted_at"
     t.index ["name"], name: "idx_stores_name"