Add cleanup for orphaned temporary upload files #829

Problem

When an upload request has an uncertain outcome, such as a timeout or connection loss after EventStore may have accepted the event, the temporary file must be retained. However, files retained this way need a later cleanup path so the temporary directory does not grow indefinitely.

Proposed solution

Add a lightweight background cleanup task for the single-server setup that periodically scans the temporary upload directory.

• Keep files referenced by pending EventStore/import-log processing.
• Move successfully processed files to the existing ProcessedDirectory.
• Move definitively failed processing files to the existing FailedDirectory.
• Treat unreferenced files older than a configurable retention period, such as 24–72 hours, as orphaned.
• Delete orphaned files or move them to a quarantine directory, with logging and metrics.
• Make the retention period and scan interval configurable.
• Ensure cleanup is safe to run repeatedly and does not interfere with active uploads.

Context

This is related to pull request #828. The upload path should retain files when EventStore acceptance is uncertain rather than deleting them immediately.