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/sync/product_sync_worker.ex

@@ -59,19 +59,34 @@ defmodule SimpleshopTheme.Sync.ProductSyncWorker do
   # Private
   # =============================================================================
 
+  # Number of concurrent product syncs (DB operations only, not API calls)
+  @max_concurrency 5
+
   defp sync_products(conn) do
     Logger.info("Starting product sync for #{conn.provider_type} (#{conn.id})")
-
     Products.update_sync_status(conn, "syncing")
 
+    try do
+      do_sync_products(conn)
+    rescue
+      e ->
+        Logger.error("Product sync crashed for #{conn.provider_type}: #{Exception.message(e)}")
+        Products.update_sync_status(conn, "failed")
+        {:error, :sync_crashed}
+    end
+  end
+
+  defp do_sync_products(conn) do
     with {:ok, provider} <- Provider.for_connection(conn),
          {:ok, products} <- provider.fetch_products(conn) do
+      Logger.info("Fetched #{length(products)} products from #{conn.provider_type}")
+
       results = sync_all_products(conn, products)
 
-      created = Enum.count(results, fn {_, _, status} -> status == :created end)
-      updated = Enum.count(results, fn {_, _, status} -> status == :updated end)
-      unchanged = Enum.count(results, fn {_, _, status} -> status == :unchanged end)
-      errors = Enum.count(results, fn result -> match?({:error, _}, result) end)
+      created = Enum.count(results, &match?({:ok, _, :created}, &1))
+      updated = Enum.count(results, &match?({:ok, _, :updated}, &1))
+      unchanged = Enum.count(results, &match?({:ok, _, :unchanged}, &1))
+      errors = Enum.count(results, &match?({:error, _}, &1))
 
       Logger.info(
         "Product sync complete for #{conn.provider_type}: " <>
@@ -89,18 +104,35 @@ defmodule SimpleshopTheme.Sync.ProductSyncWorker do
   end
 
   defp sync_all_products(conn, products) do
-    Enum.map(products, fn product_data ->
-      case sync_product(conn, product_data) do
-        {:ok, product, status} ->
-          sync_product_associations(product, product_data)
-          {:ok, product, status}
-
-        error ->
-          error
-      end
+    products
+    |> Task.async_stream(
+      fn product_data -> sync_single_product(conn, product_data) end,
+      max_concurrency: @max_concurrency,
+      timeout: 30_000,
+      on_timeout: :kill_task
+    )
+    |> Enum.map(fn
+      {:ok, result} -> result
+      {:exit, :timeout} -> {:error, :timeout}
+      {:exit, reason} -> {:error, reason}
     end)
   end
 
+  defp sync_single_product(conn, product_data) do
+    case sync_product(conn, product_data) do
+      {:ok, product, status} ->
+        sync_product_associations(product, product_data)
+        {:ok, product, status}
+
+      error ->
+        Logger.warning(
+          "Failed to sync product #{product_data[:provider_product_id]}: #{inspect(error)}"
+        )
+
+        error
+    end
+  end
+
   defp sync_product(conn, product_data) do
     attrs = %{
       provider_product_id: product_data[:provider_product_id],