Allow disabling votes per roadmap (separate from board voting permissions)

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Voting permissions are configured per board today, but the roadmap is a display layer over posts that live on those boards — so there's no way to control voting on the roadmap independently.

That blocks a common setup: users vote freely on new feedback and ideas, while the roadmap stays a read-only, informational view. Once something is planned, the prioritisation decision is already made; further votes there add noise and suggest the order is still negotiable when it isn't.

Proposal: a per-roadmap toggle "Allow voting on roadmap items", default on so nothing changes for existing workspaces. When off, the vote button is hidden on the roadmap view; existing vote counts stay visible as read-only context.

Optionally reuse the existing permission tiers (Anyone / Signed-in / Segments / Team only) instead of a plain toggle, for consistency with the board matrix.