Add Pinwheel Clause support for zone-based encounters in route groups

Allows each sub-zone within a route group to have its own independent
encounter when the Pinwheel Clause rule is enabled (default on), instead
of the entire group sharing a single encounter.

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

backend/src/app/api/games.py

@@ -80,6 +80,7 @@ async def list_game_routes(
             "game_id": route.game_id,
             "order": route.order,
             "parent_route_id": route.parent_route_id,
+            "pinwheel_zone": route.pinwheel_zone,
             "encounter_methods": methods,
         }