feat: 웹사이트 표준화 검사 도구 구현

- 4개 검사 엔진: HTML/CSS, 접근성(WCAG), SEO, 성능/보안 (총 50개 항목)
- FastAPI 백엔드 (9개 API, SSE 실시간 진행, PDF/JSON 리포트)
- Next.js 15 프론트엔드 (6개 페이지, 29개 컴포넌트, 반원 게이지 차트)
- Docker Compose 배포 (Backend:8011, Frontend:3011, MongoDB:27022, Redis:6392)
- 전체 테스트 32/32 PASS

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/engines/base.py

@@ -0,0 +1,108 @@
+"""
+BaseChecker abstract class - foundation for all inspection engines.
+"""
+
+from abc import ABC, abstractmethod
+from typing import Callable, Optional
+
+from app.models.schemas import CategoryResult, Issue, Severity, calculate_grade
+
+
+class BaseChecker(ABC):
+    """
+    Abstract base class for all inspection engines.
+    Provides progress callback mechanism and common utility methods.
+    """
+
+    def __init__(self, progress_callback: Optional[Callable] = None):
+        self.progress_callback = progress_callback
+
+    async def update_progress(self, progress: int, current_step: str) -> None:
+        """Update progress via Redis callback."""
+        if self.progress_callback:
+            await self.progress_callback(
+                category=self.category_name,
+                progress=progress,
+                current_step=current_step,
+            )
+
+    @property
+    @abstractmethod
+    def category_name(self) -> str:
+        """Category identifier (e.g., 'html_css')."""
+        pass
+
+    @abstractmethod
+    async def check(self, url: str, html_content: str, headers: dict) -> CategoryResult:
+        """Execute inspection and return results."""
+        pass
+
+    def _create_issue(
+        self,
+        code: str,
+        severity: str,
+        message: str,
+        suggestion: str,
+        element: Optional[str] = None,
+        line: Optional[int] = None,
+        wcag_criterion: Optional[str] = None,
+    ) -> Issue:
+        """Helper to create a standardized Issue object."""
+        return Issue(
+            code=code,
+            category=self.category_name,
+            severity=Severity(severity),
+            message=message,
+            element=element,
+            line=line,
+            suggestion=suggestion,
+            wcag_criterion=wcag_criterion,
+        )
+
+    def _calculate_score_by_deduction(self, issues: list[Issue]) -> int:
+        """
+        Calculate score by deduction:
+        score = 100 - (Critical*15 + Major*8 + Minor*3 + Info*1)
+        Minimum 0, Maximum 100
+        """
+        severity_weights = {
+            "critical": 15,
+            "major": 8,
+            "minor": 3,
+            "info": 1,
+        }
+        deduction = sum(
+            severity_weights.get(issue.severity.value, 0) for issue in issues
+        )
+        return max(0, 100 - deduction)
+
+    def _build_result(
+        self,
+        category: str,
+        score: int,
+        issues: list[Issue],
+        wcag_level: Optional[str] = None,
+        meta_info: Optional[dict] = None,
+        sub_scores: Optional[dict] = None,
+        metrics: Optional[dict] = None,
+    ) -> CategoryResult:
+        """Build a CategoryResult with computed severity counts."""
+        critical = sum(1 for i in issues if i.severity == Severity.CRITICAL)
+        major = sum(1 for i in issues if i.severity == Severity.MAJOR)
+        minor = sum(1 for i in issues if i.severity == Severity.MINOR)
+        info = sum(1 for i in issues if i.severity == Severity.INFO)
+
+        return CategoryResult(
+            score=score,
+            grade=calculate_grade(score),
+            total_issues=len(issues),
+            critical=critical,
+            major=major,
+            minor=minor,
+            info=info,
+            issues=issues,
+            wcag_level=wcag_level,
+            meta_info=meta_info,
+            sub_scores=sub_scores,
+            metrics=metrics,
+        )