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

database/src/main/java/com/alttd/altitudeweb/database/web_db/forms/DiscordAppeal.java

@@ -7,7 +7,7 @@ public record DiscordAppeal(
         UUID id,
         UUID uuid,
         Long discordId,
-        String username,
+        String discordUsername,
         String reason,
         Instant createdAt,
         Instant sendAt,