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>

pkg/apperror/apperror.go

@@ -0,0 +1,33 @@
+package apperror
+
+import "fmt"
+
+// AppError is a structured application error with an HTTP status code.
+type AppError struct {
+	Code    string `json:"error"`
+	Message string `json:"message"`
+	Status  int    `json:"-"`
+}
+
+func (e *AppError) Error() string { return e.Message }
+
+// New creates a new AppError.
+func New(code string, message string, status int) *AppError {
+	return &AppError{Code: code, Message: message, Status: status}
+}
+
+// Wrap creates a new AppError that wraps a cause error.
+func Wrap(code string, message string, status int, cause error) *AppError {
+	return &AppError{Code: code, Message: fmt.Sprintf("%s: %v", message, cause), Status: status}
+}
+
+// Common errors
+var (
+	ErrUnauthorized = &AppError{Code: "unauthorized", Message: "인증이 필요합니다", Status: 401}
+	ErrForbidden    = &AppError{Code: "forbidden", Message: "권한이 없습니다", Status: 403}
+	ErrNotFound     = &AppError{Code: "not_found", Message: "리소스를 찾을 수 없습니다", Status: 404}
+	ErrConflict     = &AppError{Code: "conflict", Message: "이미 존재합니다", Status: 409}
+	ErrBadRequest   = &AppError{Code: "bad_request", Message: "잘못된 요청입니다", Status: 400}
+	ErrInternal     = &AppError{Code: "internal_error", Message: "서버 오류가 발생했습니다", Status: 500}
+	ErrRateLimited  = &AppError{Code: "rate_limited", Message: "요청이 너무 많습니다", Status: 429}
+)