Delete marker from processedFallbackMessages on failure so message can be retried. Add MAX_PROCESSED_FALLBACK_MARKERS=500 with eviction to prevent unbounded Set growth.
Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)
Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>