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>

priv/repo/migrations/20260213020000_create_products_search.exs

@@ -0,0 +1,29 @@
+defmodule SimpleshopTheme.Repo.Migrations.CreateProductsSearch do
+  use Ecto.Migration
+
+  def up do
+    # Rowid mapping (FTS5 needs integer rowids, products use UUIDs)
+    execute """
+    CREATE TABLE products_search_map (
+      rowid INTEGER PRIMARY KEY AUTOINCREMENT,
+      product_id TEXT NOT NULL UNIQUE
+    )
+    """
+
+    # FTS5 virtual table — stores its own content for simple delete/reindex
+    execute """
+    CREATE VIRTUAL TABLE products_search USING fts5(
+      title,
+      category,
+      variant_info,
+      description,
+      tokenize='unicode61 remove_diacritics 2'
+    )
+    """
+  end
+
+  def down do
+    execute "DROP TABLE IF EXISTS products_search"
+    execute "DROP TABLE IF EXISTS products_search_map"
+  end
+end