fix: prune stale timestamps from active memory buckets

[Midoriya-w]

Closes #1477

Summary

Updated pruneMemoryBuckets to clean stale timestamps from active buckets instead of only deleting empty buckets.

Problem

Previously, pruning only removed keys when all timestamps had expired. Keys with at least one active timestamp retained all stale timestamps indefinitely, causing unnecessary memory growth over time.

Fix

• Filter expired timestamps during every prune pass
• Delete buckets with no active timestamps
• Update active buckets with only the remaining valid timestamps

Result

Memory buckets no longer accumulate stale timestamps, reducing memory usage and per-request filtering overhead.

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