DB를 MySQL에서 PostgreSQL로 전면 전환

- Sequelize dialect를 postgres로 변경, mysql2 대신 pg/pg-hstore 사용
- docker-compose: postgres:16-alpine + pg_isready healthcheck, 호스트 포트 5442
- 환경변수 MYSQL_PORT -> DB_PORT로 통일, .env.example 갱신

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/server.js

@@ -19,15 +19,15 @@ const swaggerJsdoc = require("swagger-jsdoc");
 const app = express();
 const PORT = process.env.PORT || 3000;
 
-// MySQL Connection using Sequelize
+// PostgreSQL Connection using Sequelize
 const sequelize = new Sequelize(
   process.env.DB_NAME || "report_db",
-  process.env.DB_USER || "root",
+  process.env.DB_USER || "postgres",
   process.env.DB_PASSWORD || "password",
   {
     host: process.env.DB_HOST || "localhost",
-    port: process.env.MYSQL_PORT || 3306,
-    dialect: "mysql",
+    port: process.env.DB_PORT || 5432,
+    dialect: "postgres",
     logging: false,
   },
 );
@@ -211,7 +211,7 @@ const LoginLog = sequelize.define(
 sequelize
   .authenticate()
   .then(() => {
-    console.log("MySQL Connected...");
+    console.log("PostgreSQL Connected...");
     // 테이블 구조가 변경되었을 경우 자동으로 반영하도록 alter: true 추가
     return sequelize.sync({ alter: true });
   })
@@ -243,7 +243,7 @@ sequelize
       );
     }
   })
-  .catch((err) => console.log("MySQL Connection/Sync Error:", err));
+  .catch((err) => console.log("PostgreSQL Connection/Sync Error:", err));
 
 // Auth Middleware
 const authenticateToken = (req, res, next) => {
@@ -900,7 +900,7 @@ app.post("/api/report", authenticateToken, async (req, res) => {
   try {
     const { companyId, ...data } = req.body;
 
-    // upsert (MySQL에서는 INSERT ... ON DUPLICATE KEY UPDATE)
+    // upsert (PostgreSQL에서는 INSERT ... ON CONFLICT DO UPDATE)
     const [report, created] = await Report.upsert({
       companyId,
       owner: req.user.username,