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/repository.go

@@ -237,6 +237,56 @@ func (r *Repository) UpdateRoomStatus(sessionName string, status RoomStatus) err
 	return result.Error
 }
 
+// TransitionRoomStatus atomically updates a room's status only if it currently matches expectedStatus.
+// Returns ErrStatusConflict if the row was not in the expected state (optimistic locking).
+func (r *Repository) TransitionRoomStatus(sessionName string, expectedStatus RoomStatus, newStatus RoomStatus, extras map[string]interface{}) error {
+	updates := map[string]interface{}{"status": newStatus}
+	for k, v := range extras {
+		updates[k] = v
+	}
+	result := r.db.Model(&BossRoom{}).
+		Where("session_name = ? AND status = ?", sessionName, expectedStatus).
+		Updates(updates)
+	if result.Error != nil {
+		return result.Error
+	}
+	if result.RowsAffected == 0 {
+		return ErrStatusConflict
+	}
+	return nil
+}
+
+// TransitionRoomStatusMulti atomically updates a room's status only if it currently matches one of the expected statuses.
+// Returns ErrStatusConflict if the row was not in any of the expected states.
+func (r *Repository) TransitionRoomStatusMulti(sessionName string, expectedStatuses []RoomStatus, newStatus RoomStatus, extras map[string]interface{}) error {
+	updates := map[string]interface{}{"status": newStatus}
+	for k, v := range extras {
+		updates[k] = v
+	}
+	result := r.db.Model(&BossRoom{}).
+		Where("session_name = ? AND status IN ?", sessionName, expectedStatuses).
+		Updates(updates)
+	if result.Error != nil {
+		return result.Error
+	}
+	if result.RowsAffected == 0 {
+		return ErrStatusConflict
+	}
+	return nil
+}
+
+// TransitionSlotStatus atomically updates a room slot's status only if it currently matches expectedStatus.
+func (r *Repository) TransitionSlotStatus(sessionName string, expectedStatus SlotStatus, newStatus SlotStatus) error {
+	result := r.db.Model(&RoomSlot{}).
+		Where("session_name = ? AND status = ?", sessionName, expectedStatus).
+		Update("status", newStatus)
+	if result.Error != nil {
+		return result.Error
+	}
+	// Slot transition failures are non-fatal — log but don't block
+	return nil
+}
+
 // GetRoomSlotsByServer returns all room slots for a given server.
 func (r *Repository) GetRoomSlotsByServer(serverID uint) ([]RoomSlot, error) {
 	var slots []RoomSlot