Make the build fit Vercel without breaking the container

Two settings were wrong for a platform build.

output: "standalone" tells Next.js to emit a self-contained server, which is
what the Dockerfile copies in — and what Vercel neither needs nor expects,
since it builds and packages the app itself. It is now conditional on the
VERCEL variable, which every build there sets, so each path gets what it
wants. Verified both ways: with VERCEL=1 no standalone directory appears,
without it one does.

/api/import declared maxDuration = 120. The free tier caps at 60 and refuses
anything higher, so the deployment would have failed on a value chosen for a
self-hosted server. Lowered, with the reason and the Pro ceiling written
next to it.

DEPLOYMENT.md now covers both paths, and says plainly that the repository
cannot be connected: git.elycon.solutions is self-hosted, and Vercel's git
integration only speaks GitHub, GitLab and Bitbucket. Deploying from the
workstation with the CLI works with any repository and is the shorter road;
mirroring to GitHub is written down as the alternative, with its cost — two
remotes to keep in step.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

app/api/import/route.ts

@@ -25,7 +25,11 @@ class Rueckabwicklung extends Error {
   }
 }
 
-export const maxDuration = 120;
+// 60 Sekunden, weil das die Obergrenze im kostenlosen Vercel-Tarif ist —
+// ein höherer Wert lässt sich dort nicht ausrollen. Auf einem eigenen Server
+// gilt die Angabe ohnehin nicht, und im Pro-Tarif liesse sie sich auf 300
+// heben, falls eine Datei mit vielen tausend Zeilen ansteht.
+export const maxDuration = 60;
 
 type Antwort = {
   ok: boolean;