perf(redis): speed up key browser scans

[yavon007]

Summary

speed up key browser scans

Refs #255

[t8y2]

✅ Well Done

• Pipeline batching — Replaces N sequential TYPE round-trips with a single redis::pipe(), which is the correct Redis optimization for bulk key metadata.
• Deferred detail loading — Removing per-key TTL / size / preview fetches from the scan path is a meaningful latency reduction, especially for large keyspaces. The get_value path still provides full details on selection.
• Clean removal — fetch_key_preview was only called from scan_keys_page; deleting it entirely avoids dead code.
• Frontend consistency — formatTtl and updated formatSize correctly render placeholder values as empty strings rather than misleading zeros.
• Vec::with_capacity and early return on empty keys are nice touches.

Verdict

Clean performance optimization with no correctness or safety regressions. LGTM 👍

[t8y2]

👍