Add --prune flag to seed command to remove stale data

Without --prune, seeds continue to only upsert (add/update).
With --prune, routes, encounters, and bosses not present in the
seed JSON files are deleted from the database.

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

backend/src/app/seeds/__main__.py

@@ -2,6 +2,7 @@
 
 Usage:
     python -m app.seeds            # Run seed
+    python -m app.seeds --prune    # Run seed and remove stale data not in seed files
     python -m app.seeds --verify   # Run seed + verification
     python -m app.seeds --export   # Export all seed data from DB to JSON files
 """
@@ -21,7 +22,8 @@ async def main():
         await export_all()
         return
 
-    await seed()
+    prune = "--prune" in sys.argv
+    await seed(prune=prune)
     if "--verify" in sys.argv:
         await verify()