Add section field to boss battle export and seed pipeline

The section field was missing from the export endpoint, seed loader
(insert + upsert), and the seed export function.

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

backend/src/app/api/export.py

@@ -143,6 +143,7 @@ async def export_game_bosses(
                 "level_cap": b.level_cap,
                 "order": b.order,
                 "location": b.location,
+                "section": b.section,
                 "sprite_url": b.sprite_url,
                 "pokemon": [
                     {