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/game.py

@@ -1,4 +1,4 @@
-from sqlalchemy import SmallInteger, String
+from sqlalchemy import ForeignKey, SmallInteger, String
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 
 from app.core.database import Base
@@ -15,10 +15,14 @@ class Game(Base):
     box_art_url: Mapped[str | None] = mapped_column(String(500))
     release_year: Mapped[int | None] = mapped_column(SmallInteger)
     color: Mapped[str | None] = mapped_column(String(7))  # Hex color e.g. #FF0000
+    version_group_id: Mapped[int | None] = mapped_column(
+        ForeignKey("version_groups.id"), index=True
+    )
 
-    routes: Mapped[list["Route"]] = relationship(back_populates="game")
+    version_group: Mapped["VersionGroup | None"] = relationship(
+        back_populates="games"
+    )
     runs: Mapped[list["NuzlockeRun"]] = relationship(back_populates="game")
-    boss_battles: Mapped[list["BossBattle"]] = relationship(back_populates="game")
 
     def __repr__(self) -> str:
         return f"<Game(id={self.id}, name='{self.name}')>"