fix(ai): 월드컵 중립 경기장 명시 — 잘못된 '홈 이점' 예측 근거 수정

AI 프롬프트가 Team A를 home으로 단정해 "홈 경기 이점" 같은 오류 근거가
생성됨. 조별리그는 중립 경기장(개최국 멕시코/미국/캐나다만 홈)임을 명시.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/services/ai.py

@@ -34,9 +34,13 @@ class MatchContext:
 
 def _prompt(ctx: MatchContext) -> str:
     return (
-        f"You are a football match analyst. Predict the result of this match.\n"
-        f"Match: {ctx.team_a} (Team A, home) vs {ctx.team_b} (Team B, away)\n"
-        f"Venue: {ctx.venue}\nKickoff: {ctx.kickoff}\n\n"
+        f"You are a football match analyst. Predict the result of this "
+        f"2026 FIFA World Cup match.\n"
+        f"Team A: {ctx.team_a}\nTeam B: {ctx.team_b}\n"
+        f"Venue: {ctx.venue}\nKickoff: {ctx.kickoff}\n"
+        f"Important: World Cup group-stage matches are played at NEUTRAL venues. "
+        f"Neither team has home advantage unless it is a host nation "
+        f"(Mexico, USA, or Canada). Do NOT claim home advantage otherwise.\n\n"
         f"Predict the final regulation-time score. "
         f"Respond with a single JSON object and nothing else, with keys:\n"
         f'  "scoreA": integer 0-9 (Team A goals),\n'