perf: 依赖数据库 CASCADE/SET NULL 替代手动清理关联表，缩短删除事务

- 批量删除移除 cleanup_key_references 手动清理，改为依赖 FK CASCADE/SET NULL
- video_tasks.key_id FK 增加 ondelete="SET NULL"，附带幂等迁移脚本
- _sync_delete 增加 statement_timeout 和任务级超时保护
- 批量导入在每次 await 前释放闲置 DB 连接，按批次提交写入避免长事务
- 前端轮询改为先查后等，首次查询不再多等一个间隔

alembic/versions/20260309_0100_a1b2c3d4e5f6_video_task_key_id_set_null_on_delete.py

@@ -0,0 +1,81 @@
+"""video_tasks.key_id: add ondelete SET NULL
+
+Revision ID: a1b2c3d4e5f6
+Revises: 2053ab8ed764
+Create Date: 2026-03-09 01:00:00.000000+00:00
+
+"""
+
+import sqlalchemy as sa
+
+from alembic import op
+
+# revision identifiers, used by Alembic.
+revision = "a1b2c3d4e5f6"
+down_revision = "2053ab8ed764"
+branch_labels = None
+depends_on = None
+
+_TABLE = "video_tasks"
+_FK_NAME = "video_tasks_key_id_fkey"
+
+
+def _fk_ondelete(table_name: str, constraint_name: str) -> str | None:
+    bind = op.get_bind()
+    result = bind.execute(
+        sa.text(
+            "SELECT rc.delete_rule "
+            "FROM information_schema.referential_constraints rc "
+            "JOIN information_schema.table_constraints tc "
+            "  ON rc.constraint_name = tc.constraint_name "
+            "WHERE tc.table_name = :table AND tc.constraint_name = :name"
+        ),
+        {"table": table_name, "name": constraint_name},
+    )
+    row = result.first()
+    return row[0] if row else None
+
+
+def _replace_fk_if_needed(
+    constraint_name: str,
+    table_name: str,
+    ref_table: str,
+    local_cols: list[str],
+    remote_cols: list[str],
+    desired_ondelete: str,
+) -> None:
+    current = _fk_ondelete(table_name, constraint_name)
+    if current and current.upper() == desired_ondelete.upper():
+        return
+    if current:
+        op.drop_constraint(constraint_name, table_name, type_="foreignkey")
+    op.create_foreign_key(
+        constraint_name,
+        table_name,
+        ref_table,
+        local_cols,
+        remote_cols,
+        ondelete=desired_ondelete,
+    )
+
+
+def upgrade() -> None:
+    _replace_fk_if_needed(
+        _FK_NAME,
+        _TABLE,
+        "provider_api_keys",
+        ["key_id"],
+        ["id"],
+        "SET NULL",
+    )
+
+
+def downgrade() -> None:
+    _replace_fk_if_needed(
+        _FK_NAME,
+        _TABLE,
+        "provider_api_keys",
+        ["key_id"],
+        ["id"],
+        "NO ACTION",
+    )