Add cache for recent sightings queries

[kpb]

Goal

• Avoid repeated eBird calls for the same query parameters.

Scope

• Implement a small cache keyed by:
  • lat,lon + radiusKm + daysBack
• Storage options:
  • in-memory map with TTL
  • file cache under WINGIT_CACHE_DIR (preferred for demo repeatability)
• Add config env var:
  • WINGIT_CACHE_DIR (optional)

Acceptance criteria

• Cache hit avoids HTTP call
• TTL expiration works
• Tests cover hit/miss behavior