feat(system): 新增操作日志表与 V2 迁移

需求 2.5.1 要求记录用户管理操作，先落表结构：

- operation_logs 表：冗余 username 保证账号删除后仍可追溯，
  user_id 外键 ON DELETE SET NULL，success 默认 '0' 与 login_logs 口径一致
- 截断常量（USERNAME/ERROR_MESSAGE/REQUEST_DATA）集中定义，供写入方引用
- 修复既有 V1 迁移与迁移模板的 downgrade：索引是外键唯一支撑索引，
  先 drop_index 会报 errno 1553，改为只 drop_table
- 同步 db/migration/V2 与 init-dev.sql，迁移版本号更新为 a45a8087831b

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

backend/alembic/script.py.mako

@@ -5,7 +5,7 @@ Revises: ${down_revision | comma,n}
 Create Date: ${create_date}
 
 """
-from typing import Sequence, Union
+from collections.abc import Sequence
 
 from alembic import op
 import sqlalchemy as sa
@@ -13,16 +13,22 @@ ${imports if imports else ""}
 
 # revision identifiers, used by Alembic.
 revision: str = ${repr(up_revision)}
-down_revision: Union[str, Sequence[str], None] = ${repr(down_revision)}
-branch_labels: Union[str, Sequence[str], None] = ${repr(branch_labels)}
-depends_on: Union[str, Sequence[str], None] = ${repr(depends_on)}
+down_revision: str | Sequence[str] | None = ${repr(down_revision)}
+branch_labels: str | Sequence[str] | None = ${repr(branch_labels)}
+depends_on: str | Sequence[str] | None = ${repr(depends_on)}
 
 
 def upgrade() -> None:
-    """Upgrade schema."""
+    """应用迁移。"""
     ${upgrades if upgrades else "pass"}
 
 
 def downgrade() -> None:
-    """Downgrade schema."""
+    """回滚迁移。
+
+    **注意（autogenerate 不会替你处理）**：生成的 `op.drop_index` 与
+    `op.drop_table` 顺序需人工检查 —— 若某索引是外键的唯一支撑索引，
+    单独 drop_index 会报 MySQL 1553（needed in a foreign key constraint）。
+    **整表回滚应只保留 `op.drop_table`**，DROP TABLE 会连带删除索引与外键。
+    """
     ${downgrades if downgrades else "pass"}