feat(block1): photo thumbnails in the inventory list

The list showed only initials even when a variety had a photo (quick-add stores
one as an encrypted BLOB). Now the avatar shows the first photo, falling back to
the initial — matching the "foto/inicial" list spec.

- VarietyListItem carries the first photo bytes; watchInventory loads them in a
  single query per emission.
- List tile renders a MemoryImage avatar when present.

Tests: repository (photo bytes surfaced, null when none) and a widget test
(avatar replaces the initial). Full suite: 37 passing, 0 skipped.

apps/app_seeds/lib/ui/inventory_list_screen.dart

@@ -122,7 +122,9 @@ class _VarietyTile extends StatelessWidget {
         ? '?'
         : trimmed.substring(0, 1).toUpperCase();
     return ListTile(
-      leading: CircleAvatar(child: Text(initial)),
+      leading: item.photo != null
+          ? CircleAvatar(backgroundImage: MemoryImage(item.photo!))
+          : CircleAvatar(child: Text(initial)),
       title: Text(item.label),
       onTap: () => context.push('/variety/${item.id}'),
     );