Cached presigned URLs may expire in long-lived sessions

[okamoto-aws]

Problem

stablePreviewUrls in useWorkspace.ts caches the first presigned URL per slide to prevent unnecessary image re-downloads during polling. However, this cache is only cleared on:

• Deck switch (new activeDeckId)
• pptxRequested trigger

If a user keeps a deck open for an extended period without triggering either condition, the cached presigned URLs will expire (S3 default is typically 15 min to 1 hour). After expiry, <img> elements will show broken images since the cached URLs are no longer valid.

Suggested Fix

Options (pick one):

1. TTL-based cache eviction - Store timestamp alongside URL, clear entries older than N minutes
2. <img onError> retry - On image load failure, evict the cached URL for that slideId and trigger a re-fetch
3. Periodic cache clear - Clear stablePreviewUrls every N minutes regardless of user action

Option 2 is probably the lightest touch since it only triggers on actual failure.

Context

Introduced in PR #15 (fix/preview-polling-redownload). The URL stabilisation cache is correct for its intended purpose; this issue tracks the edge case of long-lived sessions.

Files

• web-ui/src/hooks/useWorkspace.ts - stablePreviewUrls ref