feat(peers): 동종업종(테마) 비교 패널

- backend/app/seed/themes.py: themes_for_code(code) 헬퍼 추가 (코드 → 소속 테마들)
- backend/app/api/themes.py: GET /api/themes/peer/{code} 신규
  · 종목이 속한 모든 테마의 코드 합집합을 peer 풀로
  · 단일 SQL (ROW_NUMBER OVER PARTITION BY code) 로 21영업일 누적수익률 일괄 계산
  · 본인 / peer 평균 / 상·하위 5종목 반환
- web/lib/api.ts: PeerCompare 타입 + api.peerCompare()
- web/components/PeerComparePanel.tsx: 테마 태그 + 3-칸 요약 (본인/평균/격차) + 상·하위 5
- [code]/page.tsx 에 PeriodReturns 아래 부착. 테마 무소속 종목은 패널 자동 숨김.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

web/lib/api.ts

@@ -209,6 +209,27 @@ export type ThemeDetailResponse = {
   items: Mover[];
 };
 
+export type PeerCompareItem = {
+  code: string;
+  name: string;
+  market: string | null;
+  close: number | null;
+  base_close: number | null;
+  pct_change: number | null;
+};
+
+export type PeerCompareResponse = {
+  code: string;
+  name?: string;
+  window: number;
+  themes: { slug: string; name: string; code_count: number }[];
+  self_pct: number | null;
+  peer_avg_pct: number | null;
+  peer_count?: number;
+  top: PeerCompareItem[];
+  bottom: PeerCompareItem[];
+};
+
 export type IndexPoint = { date: string; value: number };
 
 export type IndexSeries = {
@@ -284,4 +305,8 @@ export const api = {
     getJson<ThemeDetailResponse>(
       `/api/themes/${encodeURIComponent(slug)}?limit=${limit}`,
     ),
+  peerCompare: (code: string, window = 21, limit = 5) =>
+    getJson<PeerCompareResponse>(
+      `/api/themes/peer/${encodeURIComponent(code)}?window=${window}&limit=${limit}`,
+    ),
 };