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/clients/printify.ex

@@ -154,9 +154,10 @@ defmodule SimpleshopTheme.Clients.Printify do
 
   @doc """
   List all products in a shop.
+  Printify allows a maximum of 50 products per page.
   """
   def list_products(shop_id, opts \\ []) do
-    limit = Keyword.get(opts, :limit, 100)
+    limit = Keyword.get(opts, :limit, 50)
     page = Keyword.get(opts, :page, 1)
     get("/shops/#{shop_id}/products.json?limit=#{limit}&page=#{page}")
   end