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/version_group.py

@@ -0,0 +1,21 @@
+from sqlalchemy import String
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from app.core.database import Base
+
+
+class VersionGroup(Base):
+    __tablename__ = "version_groups"
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    name: Mapped[str] = mapped_column(String(100))
+    slug: Mapped[str] = mapped_column(String(100), unique=True)
+
+    games: Mapped[list["Game"]] = relationship(back_populates="version_group")
+    routes: Mapped[list["Route"]] = relationship(back_populates="version_group")
+    boss_battles: Mapped[list["BossBattle"]] = relationship(
+        back_populates="version_group"
+    )
+
+    def __repr__(self) -> str:
+        return f"<VersionGroup(id={self.id}, name='{self.name}')>"