Cache negative proxy results to reduce battery drain

[greenart7c3]

Summary

This PR implements a negative result cache for blob proxy requests to significantly reduce battery consumption when clients repeatedly request unavailable blobs. Previously, each request for a missing blob would trigger a full crawl across all hint servers and relay lookups, even if the blob was already confirmed as unavailable moments earlier.

Key Changes

• Added notFoundCache (ConcurrentHashMap) to store recently failed blob lookups with a 60-second TTL and 256-entry limit
• Implemented proxyCacheKey() to generate cache keys from blob hash, xs servers, and author pubkeys (sorted for consistency)
• Added isRecentlyNotFound() to check cache and lazily remove expired entries
• Added markNotFound() to record failed lookups with opportunistic cleanup of expired entries when cache reaches capacity
• Integrated cache checks in both GET and HEAD request handlers to skip proxy crawls for recently failed lookups
• Updated version to 0.0.4 and added changelog entry

Implementation Details

• Cache key includes sorted lists of hint servers and author pubkeys to ensure identical requests hit the cache while different hints trigger fresh crawls
• TTL of 60 seconds balances battery savings with responsiveness to newly available blobs
• Max 256 entries prevents unbounded memory growth from one-off requests
• Expired entries are opportunistically cleaned up when the cache reaches capacity, avoiding periodic background cleanup
• Both GET and HEAD endpoints now check and update the cache consistently

https://claude.ai/code/session_014cykkDSZo6C4qTruWg3cko