Update one section of a brief's outline

Partial update of a single section. Omitted fields keep their value; null clears owned_keywords only. expected_updated_at is required and is the whole BRIEF's updated_at from GET /briefs/{id} — pass literal null if that read returned null. Send expected_heading to assert which section you believe number N is; the write is refused if the outline was reordered under you.

A brief that was imported carries its structure as an outline and no content plan. Editing one of its sections materialises the plan on first write and persists both projections, so no plan needs to exist first. A brief with neither a plan nor an outline is refused.