Bidirectional substring matching in isVisitedFeature caused unrelated
regions to be highlighted as visited (e.g. selecting Nordrhein-Westfalen
also marked Nord France due to "nord" being a substring match).
Replace the fuzzy loop with an additional exact check against the Natural
Earth name_en property to cover English-vs-native name mismatches.
Also fix Nominatim field priority to prefer state over county so
reverse-geocoded places resolve to the correct admin-1 level.
Adds integration tests ATLAS-009 through ATLAS-011 covering mark/unmark
region endpoints and user isolation.
Fixes#446