Add Pinwheel Clause support for zone-based encounters in route groups

Allows each sub-zone within a route group to have its own independent
encounter when the Pinwheel Clause rule is enabled (default on), instead
of the entire group sharing a single encounter.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/src/app/models/route.py

@@ -17,6 +17,7 @@ class Route(Base):
     parent_route_id: Mapped[int | None] = mapped_column(
         ForeignKey("routes.id", ondelete="CASCADE"), index=True, default=None
     )
+    pinwheel_zone: Mapped[int | None] = mapped_column(SmallInteger, default=None)
 
     game: Mapped["Game"] = relationship(back_populates="routes")
     route_encounters: Mapped[list["RouteEncounter"]] = relationship(