feat: workspace CRUD at platform scope + drop audit_logs

新增系统管理模块 /api/v1/platform/*:
- workspace 实体 CRUD(创建/列表/详情/更新/软删除)
- workspace 成员 CRUD(添加/列表/更新/移除)
- SystemAdminContext 依赖,仅 platform_role_id 指向 admin 角色的用户可访问
- /api/v1/auth/me 与 /auth/login 增 is_system_admin 派生字段
- 不变量:每个 workspace 至少保留一个 admin;系统管理员无法自我移除成员
- 软删除 workspace 级联软删除其成员

清理 audit_logs(无运行时写入,纯死特性):
- baseline 移除 audit_logs 建表与三索引(20 → 19 tables)
- 删除 AuditLogs 模型定义与 __init__.py 导出
- 清理 migrate_system_json / migrate_legacy_workspaces 中的 audit 写入与回填代码

API.md 增 §七系统管理,§七/§八/§九 顺延为 §八/§九/§十,附录 A/B 同步更新。

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

migrations/data/migrate_legacy_workspaces.py

@@ -6,7 +6,6 @@ import asyncio
 import hashlib
 import json
 import os
-from collections import defaultdict
 from pathlib import Path
 from typing import Any
 from urllib.parse import quote
@@ -17,7 +16,6 @@ from sqlalchemy.ext.asyncio import AsyncSession
 
 from common.db import create_database_engine, create_session_factory
 from common.db.models import (
-    AuditLogs,
     Roles,
     Users,
     WorkspaceMembers,
@@ -95,7 +93,6 @@ def new_stats() -> dict[str, int]:
         "workspaces_updated": 0,
         "workspace_members_inserted": 0,
         "workspace_members_updated": 0,
-        "audit_logs_workspace_backfilled": 0,
     }
 
 
@@ -266,44 +263,6 @@ async def migrate_members(
                 stats["workspace_members_updated"] += 1
 
 
-async def backfill_audit_workspaces(
-    session: AsyncSession,
-    source: list[LegacyWorkspace],
-    workspace_ids: dict[str, str],
-    users: dict[str, Users],
-    stats: dict[str, int],
-) -> None:
-    workspace_codes_by_username: dict[str, set[str]] = defaultdict(set)
-    for workspace in source:
-        for username in workspace.userIds:
-            workspace_codes_by_username[username].add(workspace.id)
-
-    workspace_by_user_id = {
-        users[username].user_id: workspace_ids[next(iter(codes))]
-        for username, codes in workspace_codes_by_username.items()
-        if len(codes) == 1
-    }
-
-    audit_logs = (
-        await session.scalars(
-            select(AuditLogs).where(AuditLogs.workspace_id.is_(None))
-        )
-    ).all()
-    for audit_log in audit_logs:
-        if (
-            not isinstance(audit_log.detail_json, dict)
-            or audit_log.detail_json.get("migration_source")
-            != "platform_data/system.json"
-        ):
-            continue
-        workspace_id = workspace_by_user_id.get(
-            audit_log.actor_user_id
-        )
-        if workspace_id is not None:
-            audit_log.workspace_id = workspace_id
-            stats["audit_logs_workspace_backfilled"] += 1
-
-
 async def run_migration(
     database_url: str,
     source: list[LegacyWorkspace],
@@ -339,13 +298,6 @@ async def run_migration(
                     users,
                     stats,
                 )
-                await backfill_audit_workspaces(
-                    session,
-                    source,
-                    workspace_ids,
-                    users,
-                    stats,
-                )
 
                 if apply_changes:
                     await session.commit()