Add input validation (queue name length, payload size limits), extract
status string constants to eliminate raw SQL literals, make CreateQueue
idempotent via upsert for crash recovery, fix RetentionMs default check
for negative values, cap exponential backoff at 1 hour to prevent
overflow, and replace manual mutex-based delay enforcement with
rate.Limiter from golang.org/x/time/rate for correct concurrent worker
serialization.