fix(leaderboards): prevent publication from stalling WebSocket processing

[tintinhamans]

Lobby teardown awaited external leaderboard publication on the WebSocket/lobby path.
HTTP calls, ELO updates, and retries could therefore stall processing for all connected
users.

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• Finalize match history and schedule publication in one database transaction.
• Publish from a background worker using durable state on match_history.
• Retry transient failures with persisted backoff, reject permanent 4xx failures.
• Restore per-operation DbContext usage.
• Remove Polly because retries are now handled durably by the worker.
• Add schema changes and an upgrade script.

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Apply upgrade_20260816_external_leaderboard_publication.sql before deployment.

The worker assumes a single service instance.

[STK0Cervanthes]

this would also solve #40 right?