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/types/admin.ts

@@ -64,6 +64,7 @@ export interface PaginatedPokemon {
 
 export interface CreateRouteEncounterInput {
   pokemonId: number
+  gameId: number
   encounterMethod: string
   encounterRate: number
   minLevel: number

frontend/src/types/game.ts

@@ -7,12 +7,13 @@ export interface Game {
   boxArtUrl: string | null
   releaseYear: number | null
   color: string | null
+  versionGroupId: number | null
 }
 
 export interface Route {
   id: number
   name: string
-  gameId: number
+  versionGroupId: number
   order: number
   parentRouteId: number | null
   pinwheelZone: number | null
@@ -36,6 +37,7 @@ export interface RouteEncounter {
   id: number
   routeId: number
   pokemonId: number
+  gameId: number
   encounterMethod: string
   encounterRate: number
   minLevel: number
@@ -140,7 +142,7 @@ export interface BossPokemon {
 
 export interface BossBattle {
   id: number
-  gameId: number
+  versionGroupId: number
   name: string
   bossType: BossType
   badgeName: string | null