Two concurrent sessions can both pass the IF NOT EXISTS check and then both attempt the INSERT against UX_AuditLog_EventId; the loser surfaced as SqlException 2601 (or 2627 for PK violations) and aborted the audit write. First-write-wins idempotency is the documented contract, so the race outcome is semantically a no-op — catch the two duplicate-key error numbers and log at Debug, let any other SqlException bubble.
Tests:
- InsertIfNotExistsAsync_ConcurrentDuplicateInserts_ProduceExactlyOneRow: 50 parallel inserters with the same EventId end with exactly one row and no escaped exceptions.
- QueryAsync_Keyset_SameOccurredAtUtc_TiebreaksOnEventId: four rows sharing the same OccurredAtUtc page deterministically through the (OccurredAtUtc, EventId) keyset cursor — exercises the e.OccurredAtUtc == after && e.EventId.CompareTo(afterId) < 0 branch and verifies EF Core 10's Guid.CompareTo translation against SQL Server uniqueidentifier order (deferred Bundle D reviewer recommendation).
AuditLogRepository now takes an optional ILogger<AuditLogRepository> (NullLogger default, mirrors InboundApiRepository); DI registration unchanged.
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