feat: add GitHub OAuth2 login and protect product mutations with JWT

- add /auth/login and /auth/callback (rewritten to pages/api/auth/*) for the
  GitHub Authorization Code flow, with a state cookie against login CSRF
- issue a 1h JWT after the code exchange and verify it (HS256) in the Apollo
  context; createProduct, updateProduct and deleteProduct require it
- set products.created_by from the logged-in GitHub user (upsert into users)
- preset the Authorization header in the embedded Sandbox
- add jsonwebtoken, drop unused cors, rename package to week-7

pages/api/auth/callback.js

@@ -0,0 +1,57 @@
+const jwt = require('jsonwebtoken');
+
+// Langkah 3-5 OAuth2: tukar code dengan access token, ambil profil, terbitkan JWT sendiri.
+module.exports = async function handler(req, res) {
+  const { GITHUB_CLIENT_ID, GITHUB_CLIENT_SECRET, JWT_SECRET } = process.env;
+  if (!GITHUB_CLIENT_ID || !GITHUB_CLIENT_SECRET || !JWT_SECRET) {
+    res.status(500).json({ error: 'GITHUB_CLIENT_ID / GITHUB_CLIENT_SECRET / JWT_SECRET belum diatur di environment' });
+    return;
+  }
+
+  const { code, state } = req.query;
+
+  // state sekali pakai: hapus cookie apa pun hasilnya.
+  const expectedState = req.cookies.oauth_state;
+  res.setHeader('Set-Cookie', 'oauth_state=; HttpOnly; SameSite=Lax; Path=/; Max-Age=0');
+
+  if (!code) {
+    res.status(400).json({ error: 'Parameter code tidak ada' });
+    return;
+  }
+  if (!state || !expectedState || state !== expectedState) {
+    res.status(400).json({ error: 'State tidak valid, ulangi login lewat /auth/login' });
+    return;
+  }
+
+  try {
+    const tokenRes = await fetch('https://github.com/login/oauth/access_token', {
+      method: 'POST',
+      headers: { Accept: 'application/json', 'Content-Type': 'application/json' },
+      body: JSON.stringify({
+        client_id: GITHUB_CLIENT_ID,
+        client_secret: GITHUB_CLIENT_SECRET,
+        code,
+      }),
+    });
+    const { access_token, error_description } = await tokenRes.json();
+    if (!access_token) {
+      res.status(401).json({ error: error_description || 'Gagal menukar code dengan access token' });
+      return;
+    }
+
+    const userRes = await fetch('https://api.github.com/user', {
+      headers: { Authorization: `Bearer ${access_token}`, Accept: 'application/json' },
+    });
+    if (!userRes.ok) {
+      res.status(502).json({ error: 'Gagal mengambil profil dari GitHub' });
+      return;
+    }
+    const githubUser = await userRes.json();
+
+    const token = jwt.sign({ username: githubUser.login }, JWT_SECRET, { expiresIn: '1h' });
+    res.json({ token });
+  } catch (err) {
+    console.error('[OAuth error] /auth/callback:', err);
+    res.status(500).json({ error: 'Terjadi kesalahan pada server, silakan coba lagi' });
+  }
+};