Add system check for missing database indexes (#30888)

app/lib/admin/system_check/missing_indexes_check.rb

@@ -0,0 +1,36 @@
+# frozen_string_literal: true
+
+class Admin::SystemCheck::MissingIndexesCheck < Admin::SystemCheck::BaseCheck
+  def skip?
+    !current_user.can?(:view_devops)
+  end
+
+  def pass?
+    missing_indexes.none?
+  end
+
+  def message
+    Admin::SystemCheck::Message.new(:missing_indexes_check, missing_indexes.join(', '))
+  end
+
+  private
+
+  def missing_indexes
+    @missing_indexes ||= begin
+      expected_indexes_by_table.flat_map do |table, indexes|
+        expected_indexes = indexes.map(&:name)
+        expected_indexes - existing_indexes_for(table)
+      end
+    end
+  end
+
+  def expected_indexes_by_table
+    schema_rb = Rails.root.join('db', 'schema.rb').read
+    schema_parser = Admin::Db::SchemaParser.new(schema_rb)
+    schema_parser.indexes_by_table
+  end
+
+  def existing_indexes_for(table)
+    ActiveRecord::Base.connection.indexes(table).map(&:name)
+  end
+end