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/alembic/versions/e4f5a6b7c8d9_add_boss_battles_unique_constraint.py

@@ -0,0 +1,29 @@
+"""add boss battles unique constraint
+
+Revision ID: e4f5a6b7c8d9
+Revises: d3e4f5a6b7c8
+Create Date: 2026-02-08 18:00:00.000000
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+
+
+# revision identifiers, used by Alembic.
+revision: str = 'e4f5a6b7c8d9'
+down_revision: Union[str, Sequence[str], None] = 'd3e4f5a6b7c8'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_unique_constraint(
+        'uq_boss_battles_version_group_order',
+        'boss_battles',
+        ['version_group_id', 'order'],
+    )
+
+
+def downgrade() -> None:
+    op.drop_constraint('uq_boss_battles_version_group_order', 'boss_battles', type_='unique')