Add optional specialty type field to boss battles

Gym leaders, Elite Four, and champions can now have a Pokemon type
specialty (e.g. Rock, Water). Shown as a type image badge on boss
cards in the run view, and editable via dropdown in the admin form.
Includes migration, export, and seed pipeline support.

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

backend/src/app/alembic/versions/a6b7c8d9e0f1_add_specialty_type_to_boss_battles.py

@@ -0,0 +1,26 @@
+"""add specialty_type to boss battles
+
+Revision ID: a6b7c8d9e0f1
+Revises: f5a6b7c8d9e0
+Create Date: 2026-02-08 21:00:00.000000
+
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+
+# revision identifiers, used by Alembic.
+revision: str = 'a6b7c8d9e0f1'
+down_revision: Union[str, Sequence[str], None] = 'f5a6b7c8d9e0'
+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('specialty_type', sa.String(20), nullable=True))
+
+
+def downgrade() -> None:
+    op.drop_column('boss_battles', 'specialty_type')