Sequel::Model wrapper over the invoices table. Three responsibilities:
- Invoice.build takes form params (client, currency, GEL rate, items array)
and returns an unsaved Invoice with a freshly-allocated number, normalized
line items, and computed subtotal.
- Invoice.allocate_number scans for the highest INV-<year>-NNNN and
increments. Single-process deploy means no contention; if that changes
this needs a DB-side sequence per year.
- Instance methods: paid?, status, total, total_gel for the PDF renderer
and the public landing page.
Items are stored as JSONB; qty and unit_price are kept as strings on the
way in/out so BigDecimal arithmetic stays exact through the JSON round-trip.
