feat: implement Redis session management and user authentication flow

app/gateway-api/src/auth/passwordHasher.ts

@@ -0,0 +1,13 @@
+import { createHash, randomBytes } from 'node:crypto';
+
+export interface PasswordHasher {
+    createSalt(): string;
+    hash(password: string, salt: string): string;
+}
+
+// 비밀번호 해싱은 임시 구현이므로 이후 안전한 KDF로 교체한다.
+export const createSimplePasswordHasher = (): PasswordHasher => ({
+    createSalt: () => randomBytes(16).toString('hex'),
+    hash: (password: string, salt: string) =>
+        createHash('sha256').update(`${salt}:${password}`).digest('hex'),
+});