Six endpoints: POST invoice, GET queue, GET queue/<id>, POST queue/process,
POST cancel/<id>, GET <sale_id>/pdf. Also adds cfdi_builder stub (Task 2).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Quotation and layaway endpoints were calling calculate_totals() on raw
input items without looking up unit_price/tax_rate from inventory, causing
KeyError. Added _enrich_items() helper (with customer price tier support).
Also removed non-existent discount_total column from quotations INSERT,
and made layaway complete accept a final payment for the remaining balance.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>