Bridges @Transactional upload paths to the async thumbnail pipeline.
dispatchAfterCommit registers a TransactionSynchronization so the async
task only fires after the surrounding commit (and is silently skipped
on rollback) — mirrors the AuditService.logAfterCommit pattern.
generateAsync wraps the full ThumbnailService.generate call in a 30s
watchdog so a hung PDFBox render cannot occupy a thumbnailExecutor slot
indefinitely.
Refs #307
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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