Reinstate the Vercel build settings

Reverts 61ccce5, which reverted ecbda3f. The decision came back to Vercel,
so the two platform accommodations return: output: "standalone" is
conditional on VERCEL again, and /api/import goes back to 60 seconds, the
free tier's ceiling.

The Docker path is unaffected and stays documented — including the internal
network notes and deploy/Caddyfile written in between, which remain correct
for anyone taking that road. DEPLOYMENT.md conflicted at the top and now
carries both introductions instead of one replacing the other.

Verified with VERCEL=1: builds clean and emits no standalone directory.

Stated once and recorded here rather than repeated: Vercel's Hobby plan
excludes commercial use, and this is a company's HR system. Defensible while
the database holds nothing but the 852 invented people from the seed;
Pro at $20/month is the licensed path once real personnel data is in it.

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;