Add local Mollie payment stub for development and testing

[MannyLama]

Implements a fully functional local payment provider that simulates
the Mollie payment flow without requiring external API connections.

New components:

• LocalCheckoutProvider: Implements CheckoutProvider interface with
  in-memory payment storage, automatic status management, and pattern-
  based testing support. Conditionally activated when Mollie Client
  bean is absent.

• LocalPaymentController: REST API for managing test payments with
  endpoints to list, view, and update payment statuses. Includes an
  interactive HTML checkout page that simulates the Mollie payment flow.

• Comprehensive documentation in docs/LOCAL_PAYMENT_STUB.md covering
  setup, usage, API endpoints, testing workflows, and troubleshooting.

Configuration changes:

• Updated application-local.yml to disable Mollie API key by default,
  enabling the local stub for development
• Added base-url and public-base-url configuration for local environment

Features:

• Automatic webhook triggering on payment status changes
• Support for all payment statuses (PAID, CANCELLED, REFUNDED, ONGOING)
• Pattern-based payment IDs for integration testing
• In-memory payment storage with full inspection capabilities
• Simulated checkout page with visual payment flow
• Zero external dependencies for local development

The stub provides a complete development environment for testing
payment flows across events, activities, and memberships without
needing a Mollie account or test API key.