fix: bound and cancel Nominatim requests

[atomantic]

Summary

• Bound every Nominatim attempt, including response-body parsing, to a 15-second deadline so stalled upstream connections cannot poison the shared queue.
• Propagate SSE disconnect cancellation through batch geocoding and the active fetch, without persisting cancelled work or emitting a false completion event.
• Preserve the existing 429 backoff/retry behavior and add focused lifecycle and stream-disconnect tests.

Verification

• npx vitest run tests/unit/services/geocode.service.spec.ts tests/unit/routes/mapGeocodeStream.spec.ts
• npm run build

Closes #160