Add version groups to share routes and boss battles across games

Routes and boss battles now belong to a version_group instead of
individual games, so paired versions (e.g. Red/Blue, Gold/Silver)
share the same route structure and boss battles. Route encounters
gain a game_id column to support game-specific encounter tables
within a shared route. Includes migration, updated seeds, API
changes, and frontend type updates.

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

backend/src/app/schemas/game.py

@@ -4,7 +4,7 @@ from app.schemas.base import CamelModel
 class RouteResponse(CamelModel):
     id: int
     name: str
-    game_id: int
+    version_group_id: int
     order: int
     parent_route_id: int | None = None
     pinwheel_zone: int | None = None
@@ -20,6 +20,7 @@ class GameResponse(CamelModel):
     box_art_url: str | None
     release_year: int | None
     color: str | None
+    version_group_id: int | None = None
 
 
 class RouteWithChildrenResponse(RouteResponse):