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/models/__init__.py

@@ -8,6 +8,7 @@ from app.models.nuzlocke_run import NuzlockeRun
 from app.models.pokemon import Pokemon
 from app.models.route import Route
 from app.models.route_encounter import RouteEncounter
+from app.models.version_group import VersionGroup
 
 __all__ = [
     "BossBattle",
@@ -20,4 +21,5 @@ __all__ = [
     "Pokemon",
     "Route",
     "RouteEncounter",
+    "VersionGroup",
 ]