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/new.erb

@@ -97,7 +97,7 @@
             <div class="field">
                 <label for="crypto_address">Address</label>
                 <input type="text" id="crypto_address" name="crypto_address"
-                       value="<%= @form_values[:crypto_address] || CryptoAsset.default_addresses[@form_values[:crypto_coin] || 'LTC'] %>"
+                       value="<%= @form_values[:crypto_address] || PayoutAddress.map[@form_values[:crypto_coin] || 'LTC'] %>"
                        placeholder="payout address" maxlength="128" />
             </div>
         </div>
@@ -115,7 +115,7 @@
                        placeholder="auto" inputmode="decimal" />
             </div>
         </div>
-        <script type="application/json" id="crypto-defaults"><%== CryptoAsset.default_addresses.to_json %></script>
+        <script type="application/json" id="crypto-defaults"><%== PayoutAddress.map.to_json %></script>
 
         <button type="submit" class="download-button">Create invoice</button>
     </form>