Update a portfolio's name, project, members, and/or allocation.

Members — if provided, each needs strategy_id and may carry an optional weight. With 2+ members, supply a weight for every member (summing to 1.0) or omit weight for an equal split. All members (new or existing) must belong to the portfolio's project — pass project_id to re-scope the portfolio, but every member must already belong to that project.

Allocation — pass allocation to set the policy directly, which OVERRIDES member-weight-derived allocation and is how you enable trailing_return or member_signal_rotation rotation. When allocation is omitted the existing allocation is preserved — unless members is supplied, which re-derives it from the new member weights.