refactor: extract sql_sandbox config to Mv.Config module

Centralize application-wide configuration values for better maintainability.

lib/mv/config.ex

@@ -0,0 +1,24 @@
+defmodule Mv.Config do
+  @moduledoc """
+  Configuration helper functions for the application.
+
+  Provides centralized access to configuration values to avoid
+  magic strings/atoms scattered throughout the codebase.
+  """
+
+  @doc """
+  Returns whether SQL sandbox mode is enabled.
+
+  SQL sandbox mode is typically enabled in test environments
+  to allow concurrent database access in tests.
+
+  ## Returns
+
+    - `true` if SQL sandbox is enabled
+    - `false` otherwise
+  """
+  @spec sql_sandbox?() :: boolean()
+  def sql_sandbox? do
+    Application.get_env(:mv, :sql_sandbox, false)
+  end
+end