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/api/games.py

@@ -2,7 +2,7 @@ import json
 from pathlib import Path
 
 from fastapi import APIRouter, Depends, HTTPException
-from sqlalchemy import delete, select
+from sqlalchemy import delete, select, update
 from sqlalchemy.ext.asyncio import AsyncSession
 from sqlalchemy.orm import selectinload
 
@@ -373,6 +373,13 @@ async def delete_route(
             detail="Cannot delete route with existing encounters. Delete the encounters first.",
         )
 
+    # Null out any boss battle references to this route
+    await session.execute(
+        update(BossBattle)
+        .where(BossBattle.after_route_id == route_id)
+        .values(after_route_id=None)
+    )
+
     await session.delete(route)
     await session.commit()
 

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")