v0.1.7 — Line-item optional/alternative flags

Document line items now expose two lexoffice fields the tools didn't model:

• optional — an optional position: shown with its price but not counted in the document total (Optionale Position).
• alternative — an alternative position (Alternativposition).

Added to the shared lineItemSchema, so it applies to every document create tool (invoice, quotation, credit-note, order-confirmation, delivery-note). Both are string-coercible and forwarded to the API; the model can now set them (previously they were only ever returned on reads).

Verified live: a two-position quotation with the second marked optional: true returns lineItems[1].optional === true and a total that excludes the optional position (net 100, not 150).