add FTS5 full-text product search

Adds SQLite FTS5 search index with BM25 ranking across product title,
category, variant attributes, and description. Search modal now has
live results with thumbnails, prices, and click-to-navigate. Index
rebuilds automatically after each provider sync.

Also fixes Access syntax on Product/ProductImage structs (Map.get
instead of bracket notation) which was causing crashes when real
products were loaded from the database.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/simpleshop_theme_web/live/admin/theme/index.ex

@@ -337,8 +337,8 @@ defmodule SimpleshopThemeWeb.Admin.Theme.Index do
 
   defp preview_page(%{page: :pdp} = assigns) do
     product = List.first(assigns.preview_data.products)
-    option_types = product[:option_types] || []
-    variants = product[:variants] || []
+    option_types = Map.get(product, :option_types) || []
+    variants = Map.get(product, :variants) || []
 
     {selected_options, selected_variant} =
       case variants do
@@ -468,7 +468,7 @@ defmodule SimpleshopThemeWeb.Admin.Theme.Index do
   defp build_gallery_images(product) do
     alias SimpleshopTheme.Products.ProductImage
 
-    (product[:images] || [])
+    (Map.get(product, :images) || [])
     |> Enum.sort_by(& &1.position)
     |> Enum.map(fn img -> ProductImage.direct_url(img, 1200) end)
     |> Enum.reject(&is_nil/1)