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/participation-service-api.yaml

@@ -8,15 +8,15 @@ info:
     - 당첨자 추첨 및 관리
   version: 1.0.0
   contact:
-    name: KT Event Marketing Team
-    email: support@kt-event.com
+    name: Digital Garage Team
+    email: support@kt-event-marketing.com
 
 servers:
-  - url: http://localhost:8083
+  - url: http://localhost:8084
     description: Local Development Server
-  - url: https://api-dev.kt-event.com/participation
+  - url: https://dev-api.kt-event-marketing.com/participation/v1
     description: Development Server
-  - url: https://api.kt-event.com/participation
+  - url: https://api.kt-event-marketing.com/participation/v1
     description: Production Server
 
 tags:
@@ -28,7 +28,7 @@ tags:
     description: 당첨자 추첨 및 관리
 
 paths:
-  /api/events/{eventId}/participate:
+  /events/{eventId}/participate:
     post:
       tags:
         - participation
@@ -146,7 +146,7 @@ paths:
                       code: "EVENT_NOT_ACTIVE"
                       message: "현재 참여할 수 없는 이벤트입니다"
 
-  /api/events/{eventId}/participants:
+  /events/{eventId}/participants:
     get:
       tags:
         - participant
@@ -235,7 +235,7 @@ paths:
               schema:
                 $ref: '#/components/schemas/ErrorResponse'
 
-  /api/events/{eventId}/participants/{participantId}:
+  /events/{eventId}/participants/{participantId}:
     get:
       tags:
         - participant
@@ -300,7 +300,7 @@ paths:
                       code: "PARTICIPANT_NOT_FOUND"
                       message: "참여자를 찾을 수 없습니다"
 
-  /api/events/{eventId}/draw-winners:
+  /events/{eventId}/draw-winners:
     post:
       tags:
         - winner
@@ -399,7 +399,7 @@ paths:
                       code: "ALREADY_DRAWN"
                       message: "이미 당첨자 추첨이 완료되었습니다"
 
-  /api/events/{eventId}/winners:
+  /events/{eventId}/winners:
     get:
       tags:
         - winner