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/crypto_qr.rb

@@ -0,0 +1,33 @@
+require 'rqrcode'
+require_relative 'crypto_asset'
+
+# Renders a crypto payment QR as PNG bytes for embedding in the PDF.
+# The encoded payload depends on the asset (see CryptoAsset): a payment URI
+# with amount, a bare scheme URI, or just the address.
+module CryptoQr
+  module_function
+
+  # `amount` is a BigDecimal or nil (address-only QR).
+  def uri(coin, address, amount)
+    asset = CryptoAsset[coin] or return address
+    return address unless asset[:scheme]
+    base = "#{asset[:scheme]}:#{address}"
+    return base if amount.nil? || asset[:amount_param].nil?
+    # Trim trailing zeros so the wallet shows a clean amount; keep up to 8 dp.
+    amt = amount.round(8).to_s('F').sub(/\.?0+$/, '')
+    "#{base}?#{asset[:amount_param]}=#{amt}"
+  end
+
+  # Returns PNG bytes (binary string), sized for a crisp ~120pt PDF placement.
+  def png(coin, address, amount, size: 480)
+    qr = RQRCode::QRCode.new(uri(coin, address, amount), level: :m)
+    qr.as_png(
+      bit_depth:    1,
+      border_modules: 2,
+      color:        'black',
+      fill:         'white',
+      module_px_size: 6,
+      resize_exactly_to: size
+    ).to_s
+  end
+end