Mark a visible template as a favorite for the current user.
Authenticated but not workspace-scoped — favorites are cross-workspace by design for public/starter templates and for the caller’s own private templates when they still belong to that template’s workspace.
Returns 404 when the template does not exist or is not visible to the caller. This POST intentionally enumerates success/failure for the toggle UX, while DELETE stays non-enumerating.
Response shape. workspace_id is intentionally omitted so the gallery
endpoint (which returns rows from any workspace) does not leak tenant
ownership IDs across tenants. created_by is retained as author
provenance for public/starter rows.
definition is the serialized WorkflowDefinition JSONB (same shape as
Workflow.definition) — a template is a reusable workflow snapshot.