refactor: CI/CD 파이프라인 재설계 — SSOT + 계층화된 Quality Gates

**근본적 개선사항**:

1️⃣ **Single Source of Truth (SSOT)**
   - 빌드은 한 곳에서만 실행 (_common/build-and-test.yml)
   - 아티팩트 중앙화 (GitHub Actions artifacts)
   - build.yml과 deploy-prod.yml의 중복 빌드 제거

2️⃣ **계층화된 Quality Gates**
   - Tier 1: Fast Gates (<2min) - YAML lint, secret scan, JSON validation
   - Tier 2: Critical Gates (5min) - KIS API governance, DB schema
   - Tier 3: Integration Gates (15min, 병렬) - 30+ Python validators

3️⃣ **명확한 Workflow 책임**
   - fast-validation.yml: PR 검증 (2분 내 피드백)
   - merge-to-main.yml: 전체 파이프라인 (순차 + 의존성)
   - _common/build-and-test.yml: 공유 빌드 로직

4️⃣ **Observability 강화**
   - 각 stage별 명확한 성공/실패 표시
   - Artifact 추적 가능
   - 최종 summary report 생성

**기대 효과**:
- 빌드 시간: 3-4분 → 1-2분 (-60%)
- 실패율: 90% → <10%
- 실패 원인 파악: 30분 → 5분 (-83%)
- PR 피드백: 5분 → 2분 (-60%)

**다음 작업**:
- [ ] 기존 build.yml / deploy-prod.yml 정리
- [ ] Gitea secret 설정 (QUANTENGINE_DB_PASSWORD)
- [ ] Validator 병렬화 최적화
- [ ] Notification 채널 구성

**참조**:
- docs/CICD_ANALYSIS_AND_ROADMAP.md - 상세 분석 및 로드맵

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

.gitea/workflows/_common/build-and-test.yml

@@ -0,0 +1,108 @@
+name: Reusable Build & Test
+
+on:
+  workflow_call:
+    outputs:
+      artifact-path:
+        description: "Built artifact path (quantengine-HASH.tar.gz)"
+        value: ${{ jobs.build.outputs.artifact-path }}
+      build-tag:
+        description: "Build tag for release/deployment"
+        value: ${{ jobs.build.outputs.build-tag }}
+      commit-hash:
+        description: "Git commit hash (short)"
+        value: ${{ jobs.build.outputs.commit-hash }}
+
+env:
+  DOTNET_VERSION: '10.0.x'
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    timeout-minutes: 15
+
+    outputs:
+      artifact-path: quantengine-${{ steps.metadata.outputs.commit }}.tar.gz
+      build-tag: build-${{ steps.metadata.outputs.commit }}-${{ github.run_number }}
+      commit-hash: ${{ steps.metadata.outputs.commit }}
+
+    steps:
+      - name: Checkout Code
+        uses: actions/checkout@v3
+        with:
+          fetch-depth: 0
+
+      - name: Setup .NET
+        uses: actions/setup-dotnet@v3
+        with:
+          dotnet-version: ${{ env.DOTNET_VERSION }}
+
+      - name: Generate Build Metadata
+        id: metadata
+        run: |
+          COMMIT=$(git rev-parse --short HEAD)
+          BUILD_TIME=$(date -u +'%Y-%m-%dT%H:%M:%SZ')
+
+          echo "commit=${COMMIT}" >> $GITHUB_OUTPUT
+          echo "build-time=${BUILD_TIME}" >> $GITHUB_OUTPUT
+          echo "✓ Metadata: ${COMMIT} @ ${BUILD_TIME}"
+
+      - name: Restore Dependencies
+        run: dotnet restore src/dotnet/QuantEngine.Web/QuantEngine.Web.csproj
+
+      - name: Build Release
+        run: |
+          dotnet build src/dotnet/QuantEngine.Web/QuantEngine.Web.csproj \
+            -c Release \
+            --no-restore
+
+      - name: Run Unit Tests
+        run: |
+          dotnet test src/dotnet/QuantEngine.Core.Tests/QuantEngine.Core.Tests.csproj \
+            -c Release \
+            --no-build
+
+      - name: Publish Release Package
+        run: |
+          dotnet publish src/dotnet/QuantEngine.Web/QuantEngine.Web.csproj \
+            -c Release \
+            --no-build \
+            -o ./publish
+
+      - name: Create Version Metadata
+        run: |
+          mkdir -p ./publish/wwwroot
+          cat > ./publish/wwwroot/version.json <<EOF
+          {
+            "version": "1.0.${{ github.run_number }}-${{ steps.metadata.outputs.commit }}",
+            "commit": "${{ steps.metadata.outputs.commit }}",
+            "built": "${{ steps.metadata.outputs.build-time }}",
+            "buildNumber": ${{ github.run_number }}
+          }
+          EOF
+
+      - name: Package Artifact
+        run: |
+          tar -czf quantengine-${{ steps.metadata.outputs.commit }}.tar.gz \
+            -C ./publish .
+
+          SIZE=$(du -sh quantengine-${{ steps.metadata.outputs.commit }}.tar.gz | cut -f1)
+          echo "📦 Package: quantengine-${{ steps.metadata.outputs.commit }}.tar.gz ($SIZE)"
+
+          # Verify integrity
+          tar -tzf quantengine-${{ steps.metadata.outputs.commit }}.tar.gz > /dev/null || exit 1
+          echo "✓ Package integrity verified"
+
+      - name: Upload Build Artifact
+        uses: actions/upload-artifact@v3
+        with:
+          name: quantengine-build-${{ github.run_number }}
+          path: quantengine-${{ steps.metadata.outputs.commit }}.tar.gz
+          retention-days: 5
+
+      - name: Report Build Success
+        if: success()
+        run: |
+          echo "✅ Build successful"
+          echo "   Tag: build-${{ steps.metadata.outputs.commit }}-${{ github.run_number }}"
+          echo "   Artifact: quantengine-${{ steps.metadata.outputs.commit }}.tar.gz"