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/alembic/versions/f5a6b7c8d9e0_add_section_to_boss_battles.py

@@ -0,0 +1,26 @@
+"""add section to boss battles
+
+Revision ID: f5a6b7c8d9e0
+Revises: e4f5a6b7c8d9
+Create Date: 2026-02-08 20:00:00.000000
+
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+
+# revision identifiers, used by Alembic.
+revision: str = 'f5a6b7c8d9e0'
+down_revision: Union[str, Sequence[str], None] = 'e4f5a6b7c8d9'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column('boss_battles', sa.Column('section', sa.String(100), nullable=True))
+
+
+def downgrade() -> None:
+    op.drop_column('boss_battles', 'section')