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/invoice_public.erb

@@ -12,8 +12,8 @@
         <% unless @invoice.currency == 'GEL' %>
             <dt>In GEL</dt><dd class="mono">GEL <%= '%.2f' % @invoice.total_gel %> <span class="optional">@ <%= fmt_rate(@invoice.gel_rate) %></span></dd>
         <% end %>
-        <% if @invoice.ltc? && (ltc = @invoice.ltc_amount_due) %>
-            <dt>In LTC</dt><dd class="mono"><%= fmt_ltc(ltc) %> LTC<% if @invoice.ltc_rate %> <span class="optional">@ <%= fmt_rate(@invoice.ltc_rate) %> <%= @invoice.currency %></span><% end %></dd>
+        <% if @invoice.crypto? && (amt = @invoice.crypto_amount_due) %>
+            <dt>In <%= @invoice.crypto_coin %></dt><dd class="mono"><%= fmt_crypto(amt) %> <%= @invoice.crypto_coin %><% if @invoice.crypto_rate %> <span class="optional">@ <%= fmt_rate(@invoice.crypto_rate) %> <%= @invoice.currency %></span><% end %></dd>
         <% end %>
         <dt>Status</dt><dd><span class="badge badge-<%= @invoice.status %>"><%= @invoice.status %></span></dd>
     </dl>