feat: 에러 처리 표준화 + BossRaid 낙관적 잠금

에러 표준화:
- pkg/apperror — AppError 타입, 7개 sentinel error
- pkg/middleware/error_handler — Fiber ErrorHandler 통합
- 핸들러에서 AppError 반환 시 구조화된 JSON 자동 응답

BossRaid Race Condition:
- 상태 전이 4곳 낙관적 잠금 (UPDATE WHERE status=?)
- TransitionRoomStatus/TransitionRoomStatusMulti 메서드 추가
- ErrStatusConflict sentinel error

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

internal/bossraid/model.go

@@ -1,11 +1,15 @@
 package bossraid
 
 import (
+	"errors"
 	"time"
 
 	"gorm.io/gorm"
 )
 
+// ErrStatusConflict indicates that a room's status was already changed by another request.
+var ErrStatusConflict = errors.New("방 상태가 이미 변경되었습니다")
+
 type RoomStatus string
 
 const (