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.

test/crypto_test.rb

@@ -0,0 +1,71 @@
+require_relative 'test_helper'
+
+require_relative '../lib/crypto_asset'
+require_relative '../lib/crypto_qr'
+
+class CryptoAssetTest < Minitest::Test
+  def test_registry_entries_are_complete
+    CryptoAsset::ASSETS.each do |code, asset|
+      assert asset[:name], "#{code} missing name"
+      assert asset[:coingecko], "#{code} missing coingecko id"
+    end
+  end
+
+  def test_valid_is_case_insensitive
+    assert CryptoAsset.valid?('btc')
+    assert CryptoAsset.valid?('usdt-trc20')
+    refute CryptoAsset.valid?('DOGE')
+  end
+
+  def test_default_addresses_from_json_env
+    ENV['CRYPTO_ADDRESSES'] = '{"BTC":"bc1qtest","usdt-trc20":"Ttest","DOGE":"ignored"}'
+    map = CryptoAsset.default_addresses
+    assert_equal 'bc1qtest', map['BTC']
+    assert_equal 'Ttest', map['USDT-TRC20']
+    refute map.key?('DOGE')
+  ensure
+    ENV.delete('CRYPTO_ADDRESSES')
+  end
+
+  def test_legacy_ltc_address_env_still_fills_ltc
+    ENV['LTC_ADDRESS'] = 'ltc1legacy'
+    assert_equal 'ltc1legacy', CryptoAsset.default_addresses['LTC']
+  ensure
+    ENV.delete('LTC_ADDRESS')
+  end
+
+  def test_default_addresses_survives_bad_json
+    ENV['CRYPTO_ADDRESSES'] = 'not json'
+    assert_equal({}, CryptoAsset.default_addresses)
+  ensure
+    ENV.delete('CRYPTO_ADDRESSES')
+  end
+end
+
+class CryptoQrTest < Minitest::Test
+  def test_bip21_uri_with_amount
+    assert_equal 'bitcoin:bc1qaddr?amount=0.005',
+                 CryptoQr.uri('BTC', 'bc1qaddr', BigDecimal('0.00500000'))
+    assert_equal 'litecoin:ltc1qaddr?amount=1.25',
+                 CryptoQr.uri('LTC', 'ltc1qaddr', BigDecimal('1.25'))
+  end
+
+  def test_monero_uses_tx_amount
+    assert_equal 'monero:4Aaddr?tx_amount=2.5',
+                 CryptoQr.uri('XMR', '4Aaddr', BigDecimal('2.5'))
+  end
+
+  def test_eth_scheme_without_amount
+    assert_equal 'ethereum:0xAddr', CryptoQr.uri('ETH', '0xAddr', BigDecimal('1.5'))
+  end
+
+  def test_tokens_encode_bare_address
+    assert_equal 'TAddr', CryptoQr.uri('USDT-TRC20', 'TAddr', BigDecimal('100'))
+    assert_equal 'AAddr', CryptoQr.uri('USDC-ALGO', 'AAddr', nil)
+  end
+
+  def test_png_renders
+    png = CryptoQr.png('BTC', 'bc1qexampleexampleexample', BigDecimal('0.01'))
+    assert png.start_with?("\x89PNG".b)
+  end
+end