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>

frontend/src/api/games.ts

@@ -19,6 +19,7 @@ export function getGameRoutes(gameId: number): Promise<Route[]> {
   return api.get(`/games/${gameId}/routes?flat=true`)
 }
 
-export function getRoutePokemon(routeId: number): Promise<RouteEncounterDetail[]> {
-  return api.get(`/routes/${routeId}/pokemon`)
+export function getRoutePokemon(routeId: number, gameId?: number): Promise<RouteEncounterDetail[]> {
+  const params = gameId != null ? `?game_id=${gameId}` : ''
+  return api.get(`/routes/${routeId}/pokemon${params}`)
 }