fix: address code review findings

internal/cache/preferences.go

@@ -6,13 +6,6 @@ import (
 	"time"
 )
 
-// PreferenceKey defines the structure for preference cache keys.
-type PreferenceKey struct {
-	UserID   string // user identifier
-	Kind     string // "saved_city" or "search_history"
-	CityCode string // city code for saved_city
-}
-
 // PreferenceSavedCity represents a user's saved city preference.
 type PreferenceSavedCity struct {
 	CityCode string `json:"city_code"`