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/boss.py

@@ -14,7 +14,7 @@ class BossPokemonResponse(CamelModel):
 
 class BossBattleResponse(CamelModel):
     id: int
-    game_id: int
+    version_group_id: int
     name: str
     boss_type: str
     badge_name: str | None