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/seeds/loader.py

@@ -225,6 +225,7 @@ async def upsert_bosses(
             level_cap=boss["level_cap"],
             order=boss["order"],
             location=boss["location"],
+            section=boss.get("section"),
             sprite_url=boss.get("sprite_url"),
         ).on_conflict_do_update(
             constraint="uq_boss_battles_version_group_order",
@@ -235,6 +236,7 @@ async def upsert_bosses(
                 "badge_image_url": boss.get("badge_image_url"),
                 "level_cap": boss["level_cap"],
                 "location": boss["location"],
+                "section": boss.get("section"),
                 "sprite_url": boss.get("sprite_url"),
             },
         ).returning(BossBattle.id)