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.
