Add conditional boss battle teams (variant teams by condition)

Wire up the existing condition_label column on boss_pokemon to support
variant teams throughout the UI. Boss battles can now have multiple team
configurations based on conditions (e.g., starter choice in Gen 1).

- Add condition_label to BossPokemonInput schema (frontend + backend bulk import)
- Rewrite BossTeamEditor with variant tabs (Default + named conditions)
- Add variant pill selector to BossDefeatModal team preview
- Add BossTeamPreview component to RunEncounters boss cards
- Fix MissingGreenlet error in set_boss_team via session.expunge_all()
- Fix PokemonSelector state bleed between tabs via composite React key
- Add Alembic migration for condition_label column

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

backend/src/app/alembic/versions/b7c8d9e0f1a2_add_condition_label_to_boss_pokemon.py

@@ -0,0 +1,26 @@
+"""add condition_label to boss_pokemon
+
+Revision ID: b7c8d9e0f1a2
+Revises: a6b7c8d9e0f1
+Create Date: 2026-02-08 22:00:00.000000
+
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+
+# revision identifiers, used by Alembic.
+revision: str = 'b7c8d9e0f1a2'
+down_revision: Union[str, Sequence[str], None] = 'a6b7c8d9e0f1'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column('boss_pokemon', sa.Column('condition_label', sa.String(100), nullable=True))
+
+
+def downgrade() -> None:
+    op.drop_column('boss_pokemon', 'condition_label')