Entry-fee payment & slot reservation flow

[Dejmenek]

Part of #147, depends on the model-changes and Stripe-integration sub-issues

Summary

The flow a participant goes through to pay an entry fee and provisionally claim a slot.

Tasks

• Add a way for participants to pay a tournament's entry fee and provisionally reserve a slot while payment is pending
• Free tournaments continue to work via existing join behavior
• Disable owner-added participants (AddParticipant) for paid tournaments
• Handle the existing slot-assignment race (ADR Migrate integration tests from in-memory database to Testcontainers #4) for reserved-but-unpaid slots consistently with confirmed participants
• Tests covering payment initiation, reservation, and race behavior when a tournament is nearly full