One asset per invoice from a registry (lib/crypto_asset.rb): BTC, LTC,
ETH, XMR, SOL, ALGO, USDT/USDC on ERC-20/TRC-20/BEP-20/Solana/Algorand.
Migration 003 generalizes the ltc_* columns to crypto_* + crypto_coin
(existing LTC invoices backfilled). QR payload adapts per asset: BIP21
amount URIs where supported, bare address for tokens with a network
hint on the PDF. Default addresses come from the CRYPTO_ADDRESSES
secret (JSON code=>address); LTC_ADDRESS still works as legacy.
DB.connect! opens a single Sequel connection from DATABASE_URL; DB.migrate!
runs Sequel migrations from db/migrations/.
The first migration creates the invoices table: uuid PK, unique invoice
number, client + currency fields, JSONB for line items, GEL conversion rate
captured at issue time, paid_at for the status toggle, pdf_key for the
MinIO object.
