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.

views/admin/invoices/show.erb

@@ -13,11 +13,11 @@
         <% if @invoice.paid_at %>
             <dt>Paid at</dt><dd class="mono"><%= @invoice.paid_at.utc.strftime('%Y-%m-%d %H:%M UTC') %></dd>
         <% end %>
-        <% if @invoice.ltc? %>
-            <dt>LTC</dt>
+        <% if @invoice.crypto? %>
+            <dt><%= @invoice.crypto_coin %></dt>
             <dd class="mono">
-                <% if (amt = @invoice.ltc_amount_due) %><%= amt.to_s('F').sub(/\.?0+$/, '') %> LTC<% if @invoice.ltc_rate %> @ <%= @invoice.ltc_rate.to_s('F').sub(/(\.\d*?)0+$/, '\1').sub(/\.$/, '') %> <%= @invoice.currency %><% end %><br /><% end %>
-                <span style="word-break:break-all;"><%= @invoice.ltc_address %></span>
+                <% if (amt = @invoice.crypto_amount_due) %><%= fmt_crypto(amt) %> <%= @invoice.crypto_coin %><% if @invoice.crypto_rate %> @ <%= fmt_rate(@invoice.crypto_rate) %> <%= @invoice.currency %><% end %><br /><% end %>
+                <span style="word-break:break-all;"><%= @invoice.crypto_address %></span>
             </dd>
         <% end %>
     </dl>