Rename username to discordUsername in DiscordAppeal and update references for clarity.

backend/src/main/java/com/alttd/altitudeweb/services/discord/AppealDiscord.java

@@ -55,7 +55,7 @@ public class AppealDiscord {
                 MC UUID: %s
                 Submitted: %s
                 """.formatted(
-                        safe(discordAppeal.username()),
+                        safe(discordAppeal.discordUsername()),
                         discordAppeal.discordId(),
                         safe(String.valueOf(discordAppeal.uuid())),
                         createdAt

backend/src/main/java/com/alttd/altitudeweb/services/mail/AppealMail.java

@@ -102,7 +102,7 @@ public class AppealMail {
         helper.setFrom(fromEmail);
         helper.setTo(APPEAL_EMAIL);
         helper.setReplyTo(appeal.email());
-        helper.setSubject("New Appeal Submitted - " + appeal.username());
+        helper.setSubject("New Appeal Submitted - " + appeal.discordUsername());
         return helper;
     }