use SITE_URL via helper instead of custom env var

CLAUDE.md

@@ -41,7 +41,6 @@ Auth is handled by `better-auth` via `lib/auth.ts` (server) and `lib/auth-client
 - `MONGO_URI` — MongoDB connection string
 - `ADMIN_EMAIL` — Only email allowed to log in
 - `BETTER_AUTH_SECRET` — Auth secret key
-- `BETTER_AUTH_URL` — Auth base URL (defaults to `http://localhost:3000`)
 - `GITHUB_CLIENT_ID` / `GITHUB_CLIENT_SECRET` — GitHub OAuth app (optional)
 - `NEXT_PUBLIC_GITHUB_ENABLED` — Set to enable GitHub OAuth login on the client
 - `SMTP_USER` / `SMTP_PASS` / `FROM_EMAIL` — Email via wirtanen.uberspace.de:465
@@ -64,4 +63,4 @@ Frontend dates use `dd.MM.yyyy`; backend/DB uses `yyyy-MM-dd`. Helpers in `helpe
 - `GET/PATCH /api/bookings/[uuid]` — Get or update booking
 - `GET /api/daysbooked` — Returns booked days array (JSON)
 - `GET /api/daysbooked.ics` — Returns booked days as iCal feed
-- `/api/auth/[...all]` — better-auth handler
+- `/api/auth/[...all]` — better-auth handler

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

@@ -1,3 +1,4 @@
+import { getBaseURL } from "../../../helpers/url"
 import { auth } from "../../../lib/auth"
 import type { NextApiRequest, NextApiResponse } from "next"
 
@@ -8,7 +9,7 @@ export const config = {
 }
 
 export default async function handler(req: NextApiRequest, res: NextApiResponse) {
-  const baseUrl = process.env.BETTER_AUTH_URL || "http://localhost:3000"
+  const baseUrl = getBaseURL()
   const url = new URL(req.url!, baseUrl)
 
   let body: Buffer | undefined
@@ -34,4 +35,4 @@ export default async function handler(req: NextApiRequest, res: NextApiResponse)
     res.setHeader(key, value)
   })
   res.end(await response.text())
-}
+}