7개 마이크로서비스 API 명세서 컨벤션 통일

- 공통 OpenAPI 3.0.3 컨벤션 문서 작성 (design/backend/api/API_CONVENTION.md)
- 7개 서비스 API 명세서 표준화 완료:
  * User Service (8081)
  * Event Service (8080)
  * Content Service (8082)
  * AI Service (8083)
  * Participation Service (8084)
  * Distribution Service (8085)
  * Analytics Service (8086)

주요 변경사항:
- API 경로에서 /api prefix 제거 (/api/users → /users)
- 서버 URL 패턴 통일 (Local → Dev → Prod)
- 연락처 정보 표준화 (Digital Garage Team)
- ErrorResponse 스키마 통일 (error → message, details 추가)
- securitySchemes 이름 통일 (BearerAuth)
- 포트 번호 명확히 할당

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

design/backend/api/ai-service-api.yaml

@@ -24,14 +24,16 @@ info:
 
   version: 1.0.0
   contact:
-    name: Backend Architect
-    email: architect@kt.com
+    name: Digital Garage Team
+    email: support@kt-event-marketing.com
 
 servers:
   - url: http://localhost:8083
     description: Local Development Server
-  - url: http://ai-service:8083
-    description: Kubernetes Internal Service
+  - url: https://dev-api.kt-event-marketing.com/ai/v1
+    description: Development Server
+  - url: https://api.kt-event-marketing.com/ai/v1
+    description: Production Server
 
 tags:
   - name: Health Check