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/versions/8d86e2f82860_initial_baseline_20_active_tables.py

@@ -1,4 +1,4 @@
-"""initial baseline (20 active tables)
+"""initial baseline (19 active tables)
 
 Revision ID: 8d86e2f82860
 Revises: 
@@ -21,26 +21,6 @@ depends_on: str | Sequence[str] | None = None
 def upgrade() -> None:
     """Upgrade schema."""
     # ### commands auto generated by Alembic - please adjust! ###
-    op.create_table('audit_logs',
-    sa.Column('audit_id', mysql.BIGINT(), nullable=False),
-    sa.Column('action_code', sa.String(length=128), nullable=False),
-    sa.Column('target_type', sa.String(length=64), nullable=False),
-    sa.Column('operation_status', sa.String(length=16), server_default=sa.text("'success'"), nullable=False),
-    sa.Column('created_at', mysql.DATETIME(fsp=3), server_default=sa.text('CURRENT_TIMESTAMP(3)'), nullable=False),
-    sa.Column('workspace_id', mysql.CHAR(length=26), nullable=True),
-    sa.Column('actor_user_id', mysql.CHAR(length=26), nullable=True),
-    sa.Column('target_id', sa.String(length=128), nullable=True),
-    sa.Column('client_ip', sa.String(length=45), nullable=True),
-    sa.Column('user_agent', sa.String(length=1000), nullable=True),
-    sa.Column('detail_json', sa.JSON(), nullable=True),
-    sa.Column('is_deleted', mysql.TINYINT(display_width=1), server_default=sa.text('0'), nullable=False),
-    sa.Column('deleted_at', mysql.DATETIME(fsp=3), nullable=True),
-    sa.PrimaryKeyConstraint('audit_id'),
-    comment='操作审计日志'
-    )
-    op.create_index('idx_audit_action_time', 'audit_logs', ['action_code', 'created_at'], unique=False)
-    op.create_index('idx_audit_actor_time', 'audit_logs', ['actor_user_id', 'created_at'], unique=False)
-    op.create_index('idx_audit_workspace_time', 'audit_logs', ['workspace_id', 'created_at'], unique=False)
     op.create_table('consumer_inbox',
     sa.Column('consumer_name', sa.String(length=128), nullable=False),
     sa.Column('event_id', mysql.CHAR(length=26), nullable=False),
@@ -551,8 +531,4 @@ def downgrade() -> None:
     op.drop_table('data_resources')
     op.drop_index('idx_consumer_inbox_status', table_name='consumer_inbox')
     op.drop_table('consumer_inbox')
-    op.drop_index('idx_audit_workspace_time', table_name='audit_logs')
-    op.drop_index('idx_audit_actor_time', table_name='audit_logs')
-    op.drop_index('idx_audit_action_time', table_name='audit_logs')
-    op.drop_table('audit_logs')
     # ### end Alembic commands ###