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/boss_battle.py

@@ -8,7 +8,9 @@ class BossBattle(Base):
     __tablename__ = "boss_battles"
 
     id: Mapped[int] = mapped_column(primary_key=True)
-    game_id: Mapped[int] = mapped_column(ForeignKey("games.id"), index=True)
+    version_group_id: Mapped[int] = mapped_column(
+        ForeignKey("version_groups.id"), index=True
+    )
     name: Mapped[str] = mapped_column(String(100))
     boss_type: Mapped[str] = mapped_column(String(20))  # gym_leader, elite_four, champion, rival, evil_team, other
     badge_name: Mapped[str | None] = mapped_column(String(100))
@@ -21,7 +23,9 @@ class BossBattle(Base):
     location: Mapped[str] = mapped_column(String(200))
     sprite_url: Mapped[str | None] = mapped_column(String(500))
 
-    game: Mapped["Game"] = relationship(back_populates="boss_battles")
+    version_group: Mapped["VersionGroup"] = relationship(
+        back_populates="boss_battles"
+    )
     after_route: Mapped["Route | None"] = relationship()
     pokemon: Mapped[list["BossPokemon"]] = relationship(
         back_populates="boss_battle", cascade="all, delete-orphan"