feat: CI/CD 자동 배포 워크플로우 추가

- .gitea/workflows/deploy.yml: main 머지 시 Maven 빌드 + JAR 배포
- container: maven:3.9-eclipse-temurin-17 이미지 사용
- Nexus Maven 프록시 설정 포함

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

.gitea/workflows/deploy.yml

@@ -0,0 +1,46 @@
+name: Build and Deploy API
+
+on:
+  push:
+    branches:
+      - main
+
+jobs:
+  build-and-deploy:
+    runs-on: ubuntu-latest
+    container:
+      image: maven:3.9-eclipse-temurin-17
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Configure Maven settings
+        run: |
+          mkdir -p ~/.m2
+          cat > ~/.m2/settings.xml << 'SETTINGS'
+          <settings>
+            <mirrors>
+              <mirror>
+                <id>nexus</id>
+                <mirrorOf>*</mirrorOf>
+                <url>https://nexus.gc-si.dev/repository/maven-public/</url>
+              </mirror>
+            </mirrors>
+            <servers>
+              <server>
+                <id>nexus</id>
+                <username>${{ secrets.NEXUS_USERNAME }}</username>
+                <password>${{ secrets.NEXUS_PASSWORD }}</password>
+              </server>
+            </servers>
+          </settings>
+          SETTINGS
+
+      - name: Build
+        run: mvn clean package -DskipTests -B
+
+      - name: Deploy to server
+        run: |
+          cp target/gc-guide-api-*.jar /deploy/api/app.jar
+          echo "Deployed at $(date '+%Y-%m-%d %H:%M:%S')"
+          ls -la /deploy/api/