migrate auth from next-auth to better-auth with magic link support

Replace next-auth with better-auth, adding magic link email login as
the primary auth method and GitHub OAuth as an optional social provider.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pages/api/auth/[...all].ts

@@ -0,0 +1,37 @@
+import { auth } from "../../../lib/auth"
+import type { NextApiRequest, NextApiResponse } from "next"
+
+export const config = {
+  api: {
+    bodyParser: false,
+  },
+}
+
+export default async function handler(req: NextApiRequest, res: NextApiResponse) {
+  const baseUrl = process.env.BETTER_AUTH_URL || "http://localhost:3000"
+  const url = new URL(req.url!, baseUrl)
+
+  let body: Buffer | undefined
+  if (req.method !== "GET" && req.method !== "HEAD") {
+    body = await new Promise<Buffer>((resolve, reject) => {
+      const chunks: Buffer[] = []
+      req.on("data", (chunk: Buffer) => chunks.push(chunk))
+      req.on("end", () => resolve(Buffer.concat(chunks)))
+      req.on("error", reject)
+    })
+  }
+
+  const webRequest = new Request(url.toString(), {
+    method: req.method,
+    headers: req.headers as HeadersInit,
+    body: body,
+  })
+
+  const response = await auth.handler(webRequest)
+
+  res.status(response.status)
+  response.headers.forEach((value, key) => {
+    res.setHeader(key, value)
+  })
+  res.end(await response.text())
+}

pages/api/auth/[...nextauth].ts

@@ -1,73 +0,0 @@
-import { NextApiRequest, NextApiResponse } from 'next'
-import NextAuth from 'next-auth'
-import EmailProvider from 'next-auth/providers/email'
-import GitHubProvider from 'next-auth/providers/github'
-
-import { MongoDBAdapter } from '@next-auth/mongodb-adapter'
-import { MONGO_URI } from '../../../db'
-import { MongoClient, ServerApiVersion } from 'mongodb'
-
-let client: MongoClient
-
-const ADMIN_EMAIL = process.env.ADMIN_EMAIL
-const GITHUB_USERS_GRANTED = ['111471']
-
-async function getMongoClient() {
-  if (!client) {
-    client = new MongoClient(MONGO_URI, {
-      serverApi: {
-        version: ServerApiVersion.v1,
-        strict: true,
-        deprecationErrors: true,
-      }
-    })
-    await client.connect()
-  }
-
-  return client
-}
-
-export default async function auth(req: NextApiRequest, res: NextApiResponse) {
-  return await NextAuth(req, res, {
-    secret: process.env.NEXTAUTH_SECRET,
-    adapter: MongoDBAdapter(getMongoClient()),
-    providers: [
-      GitHubProvider({
-        clientId: process.env.GITHUB_CLIENT_ID,
-        clientSecret: process.env.GITHUB_CLIENT_SECRET,
-      }),
-      EmailProvider({
-        server: {
-          host: "wirtanen.uberspace.de",
-          port: 465,
-          secure: true,
-          auth: {
-            user: process.env.SMTP_USER,
-            pass: process.env.SMTP_PASS,
-          },
-          logger: true,
-          debug: true,
-        },
-        from: process.env.FROM_EMAIL,
-      }),
-    ],
-    callbacks: {
-      async signIn({ account, email }) {
-        // if user sigin requested magic link via EmailProvider
-        if (account.provider === 'email') {
-          if (email?.verificationRequest) {
-            // only allow admins by email entered
-            return account.providerAccountId === ADMIN_EMAIL
-          }
-
-          // if user accesses with magic link, also only allow admin
-          return account.providerAccountId === ADMIN_EMAIL
-        } else if (account.provider === 'github') {
-          // only one and only one user
-          return GITHUB_USERS_GRANTED.includes(account.providerAccountId)
-        }
-        return false
-      },
-    },
-  })
-}