Move workspace membership and role validation from individual handlers
into dedicated Chi middleware. The new middleware resolves workspace ID
(from query param, X-Workspace-ID header, or URL param), validates
membership via DB, and injects the member into request context.
Handlers now read workspace ID and member from context instead of
calling requireWorkspaceMember/requireWorkspaceRole directly. This
eliminates ~17 duplicated permission checks across handlers and makes
it harder to accidentally omit access control on new routes.
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