Field-mask the scoreboard self-user read (R5)

getScoreboard loaded the full user doc just to use 5 fields. Add
getUserForScoreboard using getAll with a fieldMask so only displayName,
photoUrl, tierPoints, favoriteTeamCode, and rankSnapshot are fetched,
cutting per-request bandwidth. The short-lived me-rank cache (meRankCache,
30s) already covers the rank-count side of R5.

src/repositories/userRepository.ts

@@ -36,6 +36,44 @@ export async function getUser(uid: string): Promise<User | null> {
   return snap.exists ? (snap.data() as User) : null;
 }
 
+/** 스코어보드 "me" 계산에 필요한 최소 필드만 담는 형태. */
+export interface ScoreboardSelfUser {
+  displayName: string;
+  photoUrl?: string;
+  tierPoints?: number;
+  favoriteTeamCode?: TeamCode;
+  rankSnapshot?: RankSnapshot;
+}
+
+/**
+ * 스코어보드 응답의 "me" 계산에 필요한 필드만 fieldMask로 읽어온다.
+ * 전체 user doc(streak/티켓/notifications 등) 대신 5개 필드만 전송받아 bandwidth를 줄인다.
+ *
+ * @returns 필요한 필드 부분집합. 문서가 없으면 `null`.
+ */
+export async function getUserForScoreboard(
+  uid: string
+): Promise<ScoreboardSelfUser | null> {
+  const ref = firestore.collection(COLLECTION).doc(uid);
+  const [snap] = await firestore.getAll(ref, {
+    fieldMask: [
+      "displayName",
+      "photoUrl",
+      "tierPoints",
+      "favoriteTeamCode",
+      "rankSnapshot",
+    ],
+  });
+  if (!snap.exists) return null;
+  const data = snap.data() as Partial<User>;
+  const self: ScoreboardSelfUser = { displayName: data.displayName ?? "" };
+  if (data.photoUrl) self.photoUrl = data.photoUrl;
+  if (data.tierPoints !== undefined) self.tierPoints = data.tierPoints;
+  if (data.favoriteTeamCode) self.favoriteTeamCode = data.favoriteTeamCode;
+  if (data.rankSnapshot) self.rankSnapshot = data.rankSnapshot;
+  return self;
+}
+
 export interface ScoreboardUserEntry {
   uid: string;
   displayName: string;

src/services/scoreboardService.ts

@@ -1,7 +1,7 @@
 import { MemCache } from "../lib/memCache";
 import {
   countUsersAboveTierPoints,
-  getUser,
+  getUserForScoreboard,
 } from "../repositories/userRepository";
 import {
   readScope,
@@ -51,7 +51,7 @@ export async function getScoreboard(
   uid: string,
   type: ScoreboardType
 ): Promise<ScoreboardResponse> {
-  const user = await getUser(uid);
+  const user = await getUserForScoreboard(uid);
   if (!user) throw new HttpError(404, `user not found: ${uid}`);
 
   const teamCode = type === "team" ? user.favoriteTeamCode : undefined;