Filter out routes with no encounters for the active game

Routes are shared per version group, so game-exclusive locations (e.g.,
Black City, White Forest) appeared for both games. Now the /games/{id}/routes
endpoint excludes routes that have no encounters for the requested game,
in both flat and hierarchical modes.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.beans/nuzlocke-tracker-cftf--filter-out-routes-with-no-encounters-for-active-ga.md

@@ -0,0 +1,11 @@
+---
+# nuzlocke-tracker-cftf
+title: Filter out routes with no encounters for active game
+status: completed
+type: task
+priority: normal
+created_at: 2026-02-14T14:38:05Z
+updated_at: 2026-02-14T14:38:19Z
+---
+
+Route orders are per version group, so both games in a pair share the same route list. Routes with no encounters for the active game should be filtered out in the list_game_routes endpoint.