Create (configure) a new commitment purchase

Creates a new Commitment from the commitment creation form submission: commitment type (SAVINGS_PLAN|RESERVED), term (1yr|3yr), payment option (ALL_UPFRONT|PARTIAL_UPFRONT|NO_UPFRONT), the committed hourly rate in ten-thousandths of the currency unit per hour, and the list of covered services (service ids such as compute/storage/database). An optional recommendationId links the purchase back to the advisor recommendation that pre-filled the form. On acceptance the commitment is recorded in status QUEUED and becomes discoverable in the scoped commitment list; billing settlement later activates it. Validation: type/term/payment must be valid enum members; hourly must be a positive integer (ten-thousandths of the currency unit per hour); services must be a non-empty array of known service ids (unknown id → 422 VALIDATION_FAILED); a supplied recommendationId must reference an existing recommendation in the scope (else 404 NOT_FOUND); exceeding the scope's commitment quota maps to 409 QUOTA_EXCEEDED; caller needs owner|admin|billing, else 403. Async & state: Asynchronous — returns 202 with an Operation (PENDING → RUNNING → SUCCEEDED|FAILED). The created commitment starts in status QUEUED; when settlement completes it transitions to ACTIVE and a resource.updated SSE frame of kind billing.commitment is emitted with a patch of status, startedAt and expiresAt. Errors: 401 unauthenticated; 403 insufficient role; 404 unknown scope or referenced recommendation; 409 commitment quota exceeded for the scope; 422 field validation failure; 500 internal failure. Idempotency/Concurrency: Idempotency-Key is required; replaying the same key returns the original 202 Operation rather than purchasing twice — critical because duplicate creation incurs real spend. No If-Match applies to creation.