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())
+}