Add pessimistic locking on Competition aggregate root to close the write-skew window

[antonpampukha]

Acceptance Criteria

• New findByIdForUpdate() in CompetitionRepository with @Lock(PESSIMISTIC_WRITE) + lock timeout.
• HierarchyValidator.validateImmutabilityByCompetitionId() and CompetitionServiceImpl.changeStatus() use it instead of findById().
• It has been confirmed (via an integration test) that concurrent publish and delete operations for the last round are serialized correctly—the final state is always valid.