Add unit tests for DataCache — findNearby filtering and sorting

[GavT]

Summary

DataCache.findNearby() is the critical data path — it filters permanently closed stations and sorts by distance. It has no tests.

What to test

File: backend/src/main/kotlin/uk/co/fueller/backend/DataCache.kt

Test	Description
findNearby_excludesPermanentlyClosed	Stations with status=PermanentlyClosed are excluded
findNearby_sortsByDistance	Nearest station is first in result
findNearby_respectsRadiusLimit	No station beyond radiusMiles appears in result
findNearby_emptyCache	Returns empty list gracefully, no exception
findNearby_postcodeNotFound	Invalid postcode returns empty list

Setup

Use a pre-populated in-memory ConcurrentHashMap fixture — no real API calls needed.

Workstream: WS2 Phase 2A