feat(m3): auth & user management with session, tests, and e2e

- Custom error classes (lib/errors.ts) and signed-cookie session
  (lib/auth/session.ts, HMAC-SHA256, httpOnly+sameSite=lax)
- UserRepository + AuthService (bcrypt hashing, role/status checks,
  register/login/logout/getCurrentUser/updateProfile)
- Auth helpers (getCurrentUser), user validator, API error handler
- API routes: register (auto-login), login, logout, me, PATCH users/me,
  GET admin/migrations (admin-only guard)
- middleware.ts redirects unauthenticated page access to /login
- Screens: login (login/register toggle), profile, dashboard skeleton,
  home redirect
- vitest: exclude e2e specs, setupFiles for SESSION_SECRET
- playwright: run migrate before dev server, set SESSION_SECRET
- Unit tests (UserRepository, AuthService, session), integration
  auth-flow, e2e auth.spec (register/login/logout/protected/401)

lib/auth/session.ts

@@ -0,0 +1,93 @@
+import crypto from 'node:crypto';
+import { cookies } from 'next/headers';
+import { NextResponse } from 'next/server';
+import { SESSION_COOKIE, SESSION_MAX_AGE_SECONDS } from './constants';
+
+/**
+ * 署名付きCookieによるステートレスセッション。
+ * トークン形式: base64url(payloadJson).base64url(hmacSha256(secret, encodedPayload))
+ * payload = { uid, iat }
+ */
+
+function getSecret(): string {
+  const secret = process.env.SESSION_SECRET;
+  if (!secret) {
+    throw new Error('SESSION_SECRET is not configured');
+  }
+  return secret;
+}
+
+function sign(encodedPayload: string): string {
+  return crypto
+    .createHmac('sha256', getSecret())
+    .update(encodedPayload)
+    .digest('base64url');
+}
+
+/**
+ * ユーザーIDからセッショントークンを生成する
+ */
+export function createSessionToken(userId: number): string {
+  const payload = JSON.stringify({ uid: userId, iat: Date.now() });
+  const encoded = Buffer.from(payload, 'utf-8').toString('base64url');
+  return `${encoded}.${sign(encoded)}`;
+}
+
+/**
+ * セッショントークンを検証し、ユーザーIDを返す（不正時はnull）
+ */
+export function verifySessionToken(token: string): number | null {
+  const parts = token.split('.');
+  if (parts.length !== 2) return null;
+  const [encoded, signature] = parts;
+  const expected = sign(encoded);
+
+  const received = Buffer.from(signature);
+  const wanted = Buffer.from(expected);
+  if (received.length !== wanted.length) return null;
+  if (!crypto.timingSafeEqual(received, wanted)) return null;
+
+  try {
+    const payload = JSON.parse(
+      Buffer.from(encoded, 'base64url').toString('utf-8')
+    ) as { uid?: unknown };
+    if (typeof payload.uid !== 'number') return null;
+    return payload.uid;
+  } catch {
+    return null;
+  }
+}
+
+/**
+ * 現在のリクエスト(Server Component / Route Handler)からセッションユーザーIDを取得する
+ */
+export async function getSessionUserId(): Promise<number | null> {
+  const token = (await cookies()).get(SESSION_COOKIE)?.value;
+  if (!token) return null;
+  return verifySessionToken(token);
+}
+
+/**
+ * レスポンスにセッションCookieを設定する
+ */
+export function setSessionCookie(res: NextResponse, token: string): void {
+  res.cookies.set(SESSION_COOKIE, token, {
+    httpOnly: true,
+    sameSite: 'lax',
+    path: '/',
+    secure: process.env.NODE_ENV === 'production',
+    maxAge: SESSION_MAX_AGE_SECONDS,
+  });
+}
+
+/**
+ * レスポンスのセッションCookieを削除する（ログアウト）
+ */
+export function clearSessionCookie(res: NextResponse): void {
+  res.cookies.set(SESSION_COOKIE, '', {
+    httpOnly: true,
+    sameSite: 'lax',
+    path: '/',
+    maxAge: 0,
+  });
+}