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/tests/unit/test_models.py

@@ -11,7 +11,15 @@ from sqlalchemy.dialects import mysql
 
 from app.models import Base
 
-EXPECTED_TABLES = {"users", "roles", "user_roles", "permissions", "role_permissions", "login_logs"}
+EXPECTED_TABLES = {
+    "users",
+    "roles",
+    "user_roles",
+    "permissions",
+    "role_permissions",
+    "login_logs",
+    "operation_logs",
+}
 
 
 def _ddl(table_name: str) -> str:
@@ -101,7 +109,7 @@ class TestTimestampColumns:
     def test_带时间戳的表含ON_UPDATE子句(self, table_name: str) -> None:
         assert "ON UPDATE CURRENT_TIMESTAMP" in _ddl(table_name)
 
-    @pytest.mark.parametrize("table_name", ["login_logs"])
+    @pytest.mark.parametrize("table_name", ["login_logs", "operation_logs"])
     def test_仅追加的日志表不含ON_UPDATE子句(self, table_name: str) -> None:
         """日志是不可变记录，没有"更新时间"语义。"""
         assert "ON UPDATE CURRENT_TIMESTAMP" not in _ddl(table_name)