Add Monero (XMR) payment support

• Add MoneroPayment model for tracking XMR payments
• Add MoneroClient library for RPC communication with monero-wallet-rpc
• Add crypto payment views and routes for XMR checkout flow
• Add XMR watcher script as console entry point for payment confirmation
• Update cart/checkout views to support multiple payment methods
• Add payment method toggles in configuration (Stripe/Monero)
• Update checkout template to show Monero option when enabled
• Add crypto_checkout template for XMR payment flow

Summary by CodeRabbit

• New Features

  • Optional Monero (XMR) payments: full XMR checkout UI with live status, quotes/expiry, per-shop processor, user refund-address management, invoice payment status, and CLI tools for watching and sweeping funds.
  • Payment toggles to enable/disable Stripe and Monero.

• Bug Fixes

  • Enforced CSRF on cart actions; checkout flow now respects configured payment toggles and prevents invalid payment processing.

• Documentation

  • Expanded crypto docs, troubleshooting, README and Makefile targets for Monero workflows.

• Tests

  • New unit and integration tests for crypto payments, refund addresses, and config env-var expansion.