Enable a workflow

Moves a workflow from draft or disabled to enabled. Only enabled workflows respond to trigger events.

Enabling validates that the definition is executable — the trigger has exactly one outgoing edge (trigger-only drafts cannot be enabled), the graph is acyclic, every node is reachable from the trigger, only Scope 1 node types are present, the referenced survey and ending cards still exist, and every send_email action addressing a literal email address targets a member of this organization (an element-id to resolves to the respondent's own address and is exempt) — and then snapshots an immutable workflow version. Runs reference that snapshot via workflowVersionId, so later draft edits never change what an enabled workflow executes. Validation failures return 422 with code workflow_not_executable and structured invalid_params; calling enable in a state other than draft or disabled returns 422 with code invalid_workflow_state. Two concurrent enable requests race to publish the next version; the loser is rejected with 409 code conflict and can safely retry.