invoices: payout addresses in the database

Default crypto payout addresses lived in the CRYPTO_ADDRESSES and
LTC_ADDRESS Forgejo secrets, so changing one meant editing a secret and
redeploying. Keep them in a payout_addresses table edited at
/admin/invoices/payout-addresses (read to view, write to change, every
change audited), and prefill the new-invoice form from it.

The old env values are imported once on the first boot with an empty
table, so the current addresses carry over; after that the secrets can
be removed.

Assisted-by: Claude:opus-5

views/admin/invoices/index.erb

@@ -1,7 +1,7 @@
 <%== erb :'admin/_nav', layout: false %>
 <div class="header">
     <h1>Invoices</h1>
-    <% if can?('invoices', 'write') %><p class="tagline"><a href="/admin/invoices/new">+ New invoice</a></p><% end %>
+    <p class="tagline"><% if can?('invoices', 'write') %><a href="/admin/invoices/new">+ New invoice</a> · <% end %><a href="/admin/invoices/payout-addresses">payout addresses</a></p>
 </div>
 
 <% if @invoices.empty? %>