**Fix nickname retrieval in GuildRoutes**

Replaced `getNickname()` with `getEffectiveName()` to ensure the correct name is retrieved, falling back to the username when no nickname is set. This improves accuracy and consistency in member summaries.

src/main/java/me/bobert/netneutrality/http/server/GuildRoutes.java

@@ -471,7 +471,7 @@ public final class GuildRoutes {
         Map<String, String> summary = new LinkedHashMap<>();
         summary.put("user_id", member.getId());
         summary.put("username", member.getUser().getName());
-        summary.put("nickname", member.getNickname());
+        summary.put("nickname", member.getEffectiveName());
         return summary;
     }