feat: DB DI 전환 + download 하위 호환성 + race condition 수정

- middleware(Auth, Idempotency)를 클로저 팩토리 패턴으로 DI 전환
- database.DB/RDB 전역 변수 제거, ConnectMySQL/Redis 값 반환으로 변경
- download API X-API-Version 헤더 + 하위 호환성 규칙 문서화
- SaveGameData PlayTimeDelta 원자적 UPDATE (race condition 해소)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/player/service_test.go

@@ -98,11 +98,9 @@ func (s *testableService) SaveGameData(userID uint, data *GameDataRequest) error
 		updates["last_rot_y"] = *data.LastRotY
 	}
 	if data.PlayTimeDelta != nil {
-		profile, err := s.repo.FindByUserID(userID)
-		if err != nil {
-			return fmt.Errorf("프로필이 존재하지 않습니다")
-		}
-		updates["total_play_time"] = profile.TotalPlayTime + *data.PlayTimeDelta
+		// Mirror the real service: atomic increment via delta value.
+		// The mock UpdateStats handles this by adding to the existing value.
+		updates["total_play_time_delta"] = *data.PlayTimeDelta
 	}
 
 	if len(updates) == 0 {
@@ -211,6 +209,9 @@ func (m *mockRepo) UpdateStats(userID uint, updates map[string]interface{}) erro
 			p.LastRotY = val.(float64)
 		case "total_play_time":
 			p.TotalPlayTime = val.(int64)
+		case "total_play_time_delta":
+			// Simulates SQL: total_play_time = total_play_time + delta
+			p.TotalPlayTime += val.(int64)
 		}
 	}
 	return nil