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/models/boss_battle.py

@@ -24,6 +24,7 @@ class BossBattle(Base):
         ForeignKey("routes.id"), index=True, default=None
     )
     location: Mapped[str] = mapped_column(String(200))
+    section: Mapped[str | None] = mapped_column(String(100), default=None)
     sprite_url: Mapped[str | None] = mapped_column(String(500))
 
     version_group: Mapped["VersionGroup"] = relationship(