optimise product queries: skip image blobs, limit listing preloads, add composite index

The listing preload (images: :image) was loading the full images table row
including the data BLOB column (~3MB per page). Now only loads :id and
:source_width. Listing preloads also limited to first 2 images (primary +
hover) since product cards don't use the rest. Added composite indexes on
(visible, status, inserted_at) and (visible, status, category) to eliminate
the TEMP B-TREE sort SQLite was doing.

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

priv/repo/migrations/20260302171616_add_products_listing_indexes.exs

@@ -0,0 +1,14 @@
+defmodule Berrypod.Repo.Migrations.AddProductsListingIndexes do
+  use Ecto.Migration
+
+  def change do
+    # Composite covering index for the main listing query:
+    # WHERE visible = 1 AND status = 'active' ORDER BY inserted_at DESC
+    # Replaces the individual visible + status indexes for this pattern
+    # and eliminates the TEMP B-TREE sort SQLite was doing.
+    create index(:products, [:visible, :status, :inserted_at])
+
+    # Filtered listing by category (second most common query pattern)
+    create index(:products, [:visible, :status, :category])
+  end
+end