Revert "Make the build fit Vercel without breaking the container"

This reverts commit ecbda3f. The deployment goes to a Linux server instead,
so the two accommodations no longer earn their place: output: "standalone"
returns to unconditional, which is what the Dockerfile wants, and
/api/import goes back to 120 seconds — the free-tier ceiling that forced 60
does not apply outside a serverless platform, and a large import benefits
from the headroom.

The Vercel section in DEPLOYMENT.md goes with it.

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

next.config.ts

@@ -44,12 +44,8 @@ function contentSecurityPolicy(): string {
 
 const nextConfig: NextConfig = {
   // Emits a self-contained .next/standalone server (only the deps actually
-  // used at runtime, no full node_modules) — what the Dockerfile copies in.
-  //
-  // Auf Vercel ist das falsch: dort baut die Plattform selbst und erwartet
-  // die übliche Ausgabe. `VERCEL` setzt sie in jeder Baustrecke, die Angabe
-  // entfällt dort also von selbst — und der Docker-Weg bleibt unberührt.
-  output: process.env.VERCEL ? undefined : "standalone",
+  // used at runtime, no full node_modules) - what the Dockerfile copies in.
+  output: "standalone",
   // Baseline security headers (clickjacking, MIME-sniffing, referrer leakage,
   // browser feature access) plus the report-only CSP described above.
   async headers() {