Download badge and boss sprite images locally during export

The seed export command now downloads badge images and boss sprites
from remote URLs and stores them in frontend/public/, rewriting the
JSON URLs to local paths. Sprites are namespaced by game version
(e.g. /boss-sprites/red/brock.png) so each generation can have
its own sprite style.

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

backend/src/app/seeds/data/diamond-bosses.json

@@ -4,13 +4,13 @@
     "boss_type": "gym_leader",
     "specialty_type": "rock",
     "badge_name": "Coal Badge",
-    "badge_image_url": "https://archives.bulbagarden.net/media/upload/thumb/0/0b/Coal_Badge.png/35px-Coal_Badge.png",
+    "badge_image_url": "/badges/coal-badge.png",
     "level_cap": 14,
     "order": 1,
     "after_route_name": "Oreburgh Mine",
     "location": "Oreburgh Gym",
     "section": "Main Story",
-    "sprite_url": "https://archives.bulbagarden.net/media/upload/thumb/0/0b/Coal_Badge.png/35px-Coal_Badge.png",
+    "sprite_url": "/boss-sprites/diamond/roark.png",
     "pokemon": []
   }
 ]