Add boss seed data pipeline for export and import

Add seeder support for boss battles so new database instances come
pre-populated. Adds --export-bosses CLI flag to dump boss data from the
database to JSON seed files, and loads those files during normal seeding.

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

backend/src/app/models/boss_battle.py

@@ -1,4 +1,4 @@
-from sqlalchemy import ForeignKey, SmallInteger, String
+from sqlalchemy import ForeignKey, SmallInteger, String, UniqueConstraint
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 
 from app.core.database import Base
@@ -6,6 +6,9 @@ from app.core.database import Base
 
 class BossBattle(Base):
     __tablename__ = "boss_battles"
+    __table_args__ = (
+        UniqueConstraint("version_group_id", "order", name="uq_boss_battles_version_group_order"),
+    )
 
     id: Mapped[int] = mapped_column(primary_key=True)
     version_group_id: Mapped[int] = mapped_column(