Add after_route_name to boss battle export/seed pipeline

Exports now include after_route_name (resolved from the route FK),
and the seed loader resolves it back to an ID on import. Also adds
a draft bean for displaying encounter-less locations.

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

backend/src/app/api/export.py

@@ -127,6 +127,7 @@ async def export_game_bosses(
         .where(BossBattle.version_group_id == game.version_group_id)
         .options(
             selectinload(BossBattle.pokemon).selectinload(BossPokemon.pokemon),
+            selectinload(BossBattle.after_route),
         )
         .order_by(BossBattle.order)
     )
@@ -143,6 +144,7 @@ async def export_game_bosses(
                 "badge_image_url": b.badge_image_url,
                 "level_cap": b.level_cap,
                 "order": b.order,
+                "after_route_name": b.after_route.name if b.after_route else None,
                 "location": b.location,
                 "section": b.section,
                 "sprite_url": b.sprite_url,