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>

test/simpleshop_theme/sync/product_sync_worker_test.exs

@@ -2,6 +2,7 @@ defmodule SimpleshopTheme.Sync.ProductSyncWorkerTest do
   use SimpleshopTheme.DataCase, async: false
   use Oban.Testing, repo: SimpleshopTheme.Repo
 
+  alias SimpleshopTheme.Products
   alias SimpleshopTheme.Sync.ProductSyncWorker
 
   import SimpleshopTheme.ProductsFixtures
@@ -20,6 +21,20 @@ defmodule SimpleshopTheme.Sync.ProductSyncWorkerTest do
       assert {:cancel, :connection_disabled} =
                perform_job(ProductSyncWorker, %{provider_connection_id: conn.id})
     end
+
+    test "sets status to syncing then updates on completion or failure" do
+      conn = provider_connection_fixture(%{enabled: true})
+
+      # The job will fail because we don't have a real API connection
+      # but it should still update the status properly
+      _result = perform_job(ProductSyncWorker, %{provider_connection_id: conn.id})
+
+      # Reload the connection
+      updated_conn = Products.get_provider_connection!(conn.id)
+
+      # Status should be either "completed" or "failed", not stuck at "syncing"
+      assert updated_conn.sync_status in ["completed", "failed"]
+    end
   end
 
   describe "enqueue/1" do