Add live eBird client for recent nearby observations

[kpb]

Goal

• Fetch recent nearby observations from the eBird API (live data) and return them in the internal/types shape.

Scope

• Create internal/ebird/client.go (or similar) with a small client type.
• Support querying by:
  • location: lat,lon (initially; place name can come later)
  • radiusKm
  • daysBack
• Require token via env var:
  • WINGIT_EBIRD_TOKEN

Behavior

• If token missing: return a clear error.
• If HTTP errors: wrap with context and include status.
• Respect eBird rate limits (simple backoff or pacing is fine).

Acceptance criteria

• Unit test(s) with httptest server covering:
  • success path decoding to types.RecentObservation
  • error status code handling
• No changes to engine required.