fix(alembic): 合并 user_force_onboarding 与 comparison_idx/coupon_daily 双 head

#43 的 user_force_onboarding 迁移挂在 onboarding_completion 上，与
d4b87c5847de(comparison_status_created_idx + coupon_daily 合并、同样以
onboarding_completion 为父) 形成并列双 head，alembic upgrade head 报
Multiple head revisions，ecs1 部署会在迁移步失败未重启。

加纯合并迁移 9b894f5fff05 (Revises: d4b87c5847de, user_force_onboarding)
收敛为单 head，upgrade/downgrade no-op 不改 schema。本会话第 2 次、历史第 3
次此类问题(前为 a8c47fc4dc39 / be5e94e / f01db5d77dac)。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

alembic/versions/9b894f5fff05_merge_user_force_onboarding_and_.py

@@ -0,0 +1,26 @@
+"""merge user_force_onboarding and comparison_idx_coupon_daily heads
+
+Revision ID: 9b894f5fff05
+Revises: d4b87c5847de, user_force_onboarding
+Create Date: 2026-06-10 22:49:02.248506
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = '9b894f5fff05'
+down_revision: Union[str, Sequence[str], None] = ('d4b87c5847de', 'user_force_onboarding')
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    pass
+
+
+def downgrade() -> None:
+    pass