chore: env example 파일 정비 (루트·프론트 추가, 단독 실행 예시 제거)

- 루트 .env.{development,production}.example 신규 (비밀값 placeholder 마스킹)
- frontend .env.{development,production}.example 신규 (docker build arg 주입 설명)
- backend example 의 단독 실행(non-Docker) 예시 블록 제거

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.env.development.example

@@ -0,0 +1,39 @@
+# ==========================================
+# 프로젝트 전역 설정
+# ==========================================
+PROJECT_NAME=comrelay
+APP_ENV=development
+BUILD_MODE=development
+FRONTEND_DIR=frontend
+BACKEND_DIR=backend
+
+# ==========================================
+# 포트 설정
+# ==========================================
+FRONTEND_PORT=5273
+BACKEND_PORT=3100
+WEBSOCKET_PORT=3101
+DB_PORT=5532
+REDIS_PORT=6479
+
+# ==========================================
+# Database 설정
+# ==========================================
+DB_IMAGE=postgres:15-alpine
+DB_USER=dev_ttipo
+DB_PASSWORD=change-me-db-password
+DB_NAME=comrelay_db
+DB_DATA_DIR=./data/postgres
+
+# ==========================================
+# Redis 설정
+# ==========================================
+REDIS_IMAGE=redis:7-alpine
+REDIS_PASSWORD=change-me-redis-password
+REDIS_DATA_DIR=./data/redis
+
+# ==========================================
+# API 경로 설정
+# ==========================================
+BACKEND_API_URL=http://localhost:3100/api
+CORS_ORIGIN=http://localhost:5273