feat(themes): 테마 인덱스/상세 (11 카테고리 · 111 종목)

- seed/themes.py: 반도체/2차전지/바이오/게임/엔터/자동차/금융/조선·방산/플랫폼·AI/유통·소비재/에너지 정적 매핑
- /api/themes (인덱스) + /api/themes/{slug} (종목 카드 + 최신 종가/등락)
- 홈에 ThemeTiles 그리드, /themes/{slug} 상세 페이지

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

backend/app/main.py

@@ -15,6 +15,7 @@ from app.api.news import router as news_router
 from app.api.predict import router as predict_router
 from app.api.refresh import router as refresh_router
 from app.api.symbols import router as symbols_router
+from app.api.themes import router as themes_router
 from app.config import settings
 from app.db.connection import get_engine, ping as db_ping
 from app.fetch import dart as dart_mod
@@ -104,6 +105,7 @@ app.include_router(predict_router)
 app.include_router(metrics_router)
 app.include_router(news_router)
 app.include_router(markets_router)
+app.include_router(themes_router)
 
 
 def _resolved_device() -> str: