Fix route deletion failing due to FK constraint violations

Route deletion failed with two integrity errors:
1. route_encounters had no cascade, so SQLAlchemy tried to NULL
   the non-nullable route_id instead of deleting the rows
2. boss_battles.after_route_id referenced the route being deleted

Added cascade="all, delete-orphan" to Route.route_encounters and
nulled out boss battle after_route_id references before deletion.

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

backend/src/app/models/route.py

@@ -25,7 +25,7 @@ class Route(Base):
 
     version_group: Mapped["VersionGroup"] = relationship(back_populates="routes")
     route_encounters: Mapped[list["RouteEncounter"]] = relationship(
-        back_populates="route"
+        back_populates="route", cascade="all, delete-orphan"
     )
     encounters: Mapped[list["Encounter"]] = relationship(back_populates="route")