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_rate.rb

@@ -0,0 +1,59 @@
+require 'net/http'
+require 'json'
+require 'uri'
+require 'bigdecimal'
+require_relative 'crypto_asset'
+
+# Fetches the current price of a CryptoAsset from CoinGecko (free, no API key).
+#
+# CoinGecko's simple/price supports usd and eur directly. GEL is derived from
+# the invoice's own captured gel_rate (USD->GEL): price_gel = price_usd * gel_rate.
+module CryptoRate
+  ENDPOINT = 'https://api.coingecko.com/api/v3/simple/price'.freeze
+  SUPPORTED = %w[USD EUR].freeze
+  TIMEOUT = 6 # seconds
+
+  class Error < StandardError; end
+
+  module_function
+
+  # Returns a BigDecimal: the price of 1 unit of `coin` in `currency`.
+  # `gel_rate` (USD->GEL) is required only when currency == 'GEL'.
+  def fetch(coin, currency, gel_rate: nil)
+    asset = CryptoAsset[coin] or raise Error, "Unknown coin: #{coin}"
+    id = asset[:coingecko]
+    currency = currency.to_s.upcase
+
+    if currency == 'GEL'
+      raise Error, 'gel_rate required to derive the GEL price' if gel_rate.nil?
+      usd = fetch_simple(id, 'usd')
+      (usd * BigDecimal(gel_rate.to_s)).round(8)
+    elsif SUPPORTED.include?(currency)
+      fetch_simple(id, currency.downcase).round(8)
+    else
+      raise Error, "Unsupported currency: #{currency}"
+    end
+  end
+
+  def fetch_simple(id, vs)
+    uri = URI(ENDPOINT)
+    uri.query = URI.encode_www_form(ids: id, vs_currencies: vs)
+
+    res = Net::HTTP.start(uri.host, uri.port, use_ssl: true,
+                          open_timeout: TIMEOUT, read_timeout: TIMEOUT) do |http|
+      http.get(uri.request_uri, 'Accept' => 'application/json')
+    end
+
+    raise Error, "CoinGecko HTTP #{res.code}" unless res.is_a?(Net::HTTPSuccess)
+
+    data = JSON.parse(res.body)
+    price = data.dig(id, vs)
+    raise Error, "No price for #{id}/#{vs} in response" if price.nil?
+
+    BigDecimal(price.to_s)
+  rescue JSON::ParserError => e
+    raise Error, "Bad JSON from CoinGecko: #{e.message}"
+  rescue Net::OpenTimeout, Net::ReadTimeout
+    raise Error, 'CoinGecko timed out'
+  end
+end