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/ltc_rate.rb

@@ -1,57 +0,0 @@
-require 'net/http'
-require 'json'
-require 'uri'
-require 'bigdecimal'
-
-# Fetches the current Litecoin price from CoinGecko (free, no API key).
-#
-# CoinGecko's simple/price supports usd and eur directly. GEL is not a
-# CoinGecko vs_currency we rely on, so GEL is derived from the invoice's own
-# captured gel_rate (USD->GEL): ltc_in_gel = ltc_in_usd * gel_rate.
-module LtcRate
-  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 LTC in `currency`.
-  # `gel_rate` (USD->GEL) is required only when currency == 'GEL'.
-  def fetch(currency, gel_rate: nil)
-    currency = currency.to_s.upcase
-
-    if currency == 'GEL'
-      raise Error, 'gel_rate required to derive LTC/GEL' if gel_rate.nil?
-      usd = fetch_simple('usd')
-      (usd * BigDecimal(gel_rate.to_s)).round(8)
-    elsif SUPPORTED.include?(currency)
-      fetch_simple(currency.downcase).round(8)
-    else
-      raise Error, "Unsupported currency: #{currency}"
-    end
-  end
-
-  def fetch_simple(vs)
-    uri = URI(ENDPOINT)
-    uri.query = URI.encode_www_form(ids: 'litecoin', 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('litecoin', vs)
-    raise Error, "No price for #{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