[ chebizarro/nostrc ] feat: add avatar cache with LRU eviction and configurable memory limits
Add comprehensive avatar caching system with memory and disk tiers. Implement LRU eviction with configurable capacity via GNOSTR_AVATAR_MEM_CAP (default 200) and GNOSTR_AVATAR_SIZE (default 96px) environment variables. Use GdkPixbuf for scaled decoding to reduce memory usage. Add retry logic with exponential backoff for LMDB reader slot contention (reduce ingester threads to 1). Implement storage_ndb_get_note_by_id_nont