Move registration to async Producer pattern

[steeevin88]

CONTEXT

• RegisterForEvent handler was doing synchronous seat reservation via Redis and immediate DB acceptance inline
• this meant the HTTP handler was directly coupled to capacity-checking and acceptance logic
• moving to an async producer pattern decouples the handler from business logic and lets a consumer handle seat reservation + acceptance

CHANGES

• replace inline TryTakeEventSeat / MarkRegistrationAccepted / ReleaseEventSeat flow with producer.PublishRegistration
• status code changed from 201 Created to 202 Accepted to reflect async processing
• error messages updated to match the new enqueue-based flow

[c1ndytat]

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