fix: 批次1爆盘根因 — alembic 独立事务 + 5日期字段合并单条ALTER

生产 123 万行表(9.8GB)跑批次1时磁盘满回滚(2026-08-10)。
根因二：① alembic 无 transaction_per_migration，批次1的 01+02+03 全在一个
事务，JSON→JSONB 重写空间不释放、与后续重写叠加；② 1421ea169bb6 循环逐个
ALTER 5 个日期字段 = 5 次整表重写，同一事务累积。
修复：alembic.ini 开启 transaction_per_migration（每迁移独立提交，前序空间
释放）；1421ea169bb6 合并 5 列为单条 ALTER（一次重写）。
本地 DB 已在 head 不受影响，heads 仍单一头 55105f0bb1d7。

backend/alembic/alembic.ini

@@ -1,6 +1,12 @@
 [alembic]
 script_location = alembic
 sqlalchemy.url = postgresql://scilit:scilit_dev@localhost:5432/scilit
+# Commit each migration in its own transaction (default: whole upgrade in one).
+# Rewrite-heavy migrations (JSON->JSONB / ALTER COLUMN TYPE DATE) on the
+# 1.23M-row prod table peak at huge disk usage; one shared txn accumulates
+# them -> disk-full (2026-08-10 batch 1 incident root cause).
+# Independent commits release space early.
+transaction_per_migration = true
 
 [loggers]
 keys = root,sqlalchemy,alembic

backend/alembic/versions/1421ea169bb6_change_5_date_fields_to_date.py

@@ -21,19 +21,19 @@ DATE_FIELDS = ["pubmed_revised", "date_completed", "meshed_date", "create_date",
 
 
 def upgrade() -> None:
-    for col in DATE_FIELDS:
-        op.alter_column("global_literature", col,
-            existing_type=postgresql.TIMESTAMP(timezone=True),
-            type_=sa.Date(),
-            existing_nullable=True,
-            postgresql_using=f"{col}::date",
+    # 合并为单条 ALTER：5 列一次全表重写。
+    # 原实现循环逐个 ALTER = 5 次整表重写，同一事务内空间峰值叠加，
+    # 生产 123 万行表（~9.8GB）会爆盘（2026-08-10 批次 1 事故根因）。
+    _cols = ", ".join(
+        f'ALTER COLUMN "{col}" TYPE DATE USING "{col}"::date'
+        for col in DATE_FIELDS
     )
+    op.execute(f"ALTER TABLE global_literature {_cols}")
 
 
 def downgrade() -> None:
-    for col in reversed(DATE_FIELDS):
-        op.alter_column("global_literature", col,
-            existing_type=sa.Date(),
-            type_=postgresql.TIMESTAMP(timezone=True),
-            existing_nullable=True,
+    _cols = ", ".join(
+        f'ALTER COLUMN "{col}" TYPE TIMESTAMP WITH TIME ZONE'
+        for col in reversed(DATE_FIELDS)
     )
+    op.execute(f"ALTER TABLE global_literature {_cols}")