Add section field to boss battles for run progression dividers

Adds a nullable `section` column to boss battles (e.g. "Main Story",
"Endgame") with dividers rendered in the run view between sections.
Admin UI gets a Section column in the table and a text input in the form.

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

backend/src/app/schemas/boss.py

@@ -23,6 +23,7 @@ class BossBattleResponse(CamelModel):
     order: int
     after_route_id: int | None
     location: str
+    section: str | None
     sprite_url: str | None
     pokemon: list[BossPokemonResponse] = []
 
@@ -48,6 +49,7 @@ class BossBattleCreate(CamelModel):
     order: int
     after_route_id: int | None = None
     location: str
+    section: str | None = None
     sprite_url: str | None = None
 
 
@@ -60,6 +62,7 @@ class BossBattleUpdate(CamelModel):
     order: int | None = None
     after_route_id: int | None = None
     location: str | None = None
+    section: str | None = None
     sprite_url: str | None = None
 
 
@@ -75,6 +78,15 @@ class BossResultCreate(CamelModel):
     attempts: int = 1
 
 
+class BossReorderItem(CamelModel):
+    id: int
+    order: int
+
+
+class BossReorderRequest(CamelModel):
+    bosses: list[BossReorderItem]
+
+
 class BossResultUpdate(CamelModel):
     result: str | None = None
     attempts: int | None = None