Make ReadModelDomainEventHandler idempotent for MerchantFeeSettledEvent duplicate inserts #1872

Background\n\nTransactionProcessor#1864 shows a UniqueConstraintException while handling MerchantFeeSettledEvent in ReadModelDomainEventHandler. The observed failure is a unique constraint violation coming from the SQL read-model path.\n\nRelevant code:\n- TransactionProcessor.BusinessLogic/EventHandling/ReadModelDomainEventHandler.cs\n- TransactionProcessor.Repository/ITransactionProcessorReadModelRepository.cs\n\n## Problem\n\nThe event handling path appears to be inserting a record that already exists, or otherwise re-processing a domain event without an idempotent guard. That turns a data consistency condition into a failed event-handling run.\n\n## Scope\n\n- Review the MerchantFeeSettledEvent / related read-model insert path for duplicate handling.\n- Make the handler or repository idempotent where the event is a replay or duplicate.\n- Add regression coverage for the duplicate/unique-constraint case.\n\n## Acceptance criteria\n\n- Duplicate processing of the merchant-fee-settled read-model path does not throw a unique-constraint exception.\n- The handler logs or returns a clear duplicate/replay outcome instead of failing the whole event-processing path.\n- A regression test covers the duplicate insert case.\n\n## Traceability\n\n- Originating issue: TransactionProcessor#1864