feat: add admin provider setup UI with improved product sync

- Add /admin/providers LiveView for connecting and managing POD providers
- Implement pagination for Printify API (handles all products, not just first page)
- Add parallel processing (5 concurrent) for faster product sync
- Add slug-based fallback matching when provider_product_id changes
- Add error recovery with try/rescue to prevent stuck sync status
- Add checksum-based change detection to skip unchanged products
- Add upsert tests covering race conditions and slug matching
- Add Printify provider tests
- Document Printify integration research (product identity, order risks,
  open source vs managed hosting implications)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

lib/simpleshop_theme/providers/printify.ex

@@ -41,11 +41,7 @@ defmodule SimpleshopTheme.Providers.Printify do
     else
       with api_key when is_binary(api_key) <- ProviderConnection.get_api_key(conn),
            :ok <- set_api_key(api_key),
-           {:ok, response} <- Client.list_products(shop_id) do
-        products =
-          response["data"]
-          |> Enum.map(&normalize_product/1)
-
+           {:ok, products} <- fetch_all_products(shop_id) do
         {:ok, products}
       else
         nil -> {:error, :no_api_key}
@@ -54,6 +50,33 @@ defmodule SimpleshopTheme.Providers.Printify do
     end
   end
 
+  # Fetches all products by paginating through the API
+  defp fetch_all_products(shop_id) do
+    fetch_products_page(shop_id, 1, [])
+  end
+
+  defp fetch_products_page(shop_id, page, acc) do
+    case Client.list_products(shop_id, page: page) do
+      {:ok, response} ->
+        products = Enum.map(response["data"] || [], &normalize_product/1)
+        all_products = acc ++ products
+
+        current_page = response["current_page"] || page
+        last_page = response["last_page"] || 1
+
+        if current_page < last_page do
+          # Small delay to be nice to rate limits (600/min = 10/sec)
+          Process.sleep(100)
+          fetch_products_page(shop_id, page + 1, all_products)
+        else
+          {:ok, all_products}
+        end
+
+      {:error, _} = error ->
+        error
+    end
+  end
+
   @impl true
   def submit_order(%ProviderConnection{config: config} = conn, order) do
     shop_id = config["shop_id"]