Fixed shelf covers being stored as 'cover_book'

Are now stored as 'cover_bookshelf' as expected.
Added a migrate to alter existing shelf cover image types.

database/migrations/2022_09_02_082910_fix_shelf_cover_image_types.php

@@ -0,0 +1,42 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Support\Facades\DB;
+
+class FixShelfCoverImageTypes extends Migration
+{
+    /**
+     * Run the migrations.
+     *
+     * @return void
+     */
+    public function up()
+    {
+        // This updates the 'type' field for images, uploaded as shelf cover images,
+        // to be cover_bookshelf instead of cover_book.
+        // This does not fix their paths, since fixing that requires a more complicated operation,
+        // but their path does not affect functionality at time of this fix.
+
+        $shelfImageIds = DB::table('bookshelves')
+            ->whereNotNull('image_id')
+            ->pluck('image_id')
+            ->values()->all();
+
+        DB::table('images')
+            ->where('type', '=', 'cover_book')
+            ->whereIn('id', $shelfImageIds)
+            ->update(['type' => 'cover_bookshelf']);
+    }
+
+    /**
+     * Reverse the migrations.
+     *
+     * @return void
+     */
+    public function down()
+    {
+        DB::table('images')
+            ->where('type', '=', 'cover_bookshelf')
+            ->update(['type' => 'cover_book']);
+    }
+}