invoices: multi-crypto payment support

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.

lib/fmt.rb

@@ -13,8 +13,8 @@ module Fmt
     "#{group(int)}.#{frac}"
   end
 
-  # LTC amount: up to 8 dp, trailing zeros trimmed, no padding.
-  def ltc(value)
+  # Crypto amount: up to 8 dp, trailing zeros trimmed, no padding.
+  def crypto(value)
     BigDecimal(value.to_s).round(8).to_s('F').sub(/(\.\d*?)0+$/, '\1').sub(/\.$/, '')
   end