Stop pretending Vercel is an option

It was never used. The repository lives on a self-hosted Gitea, which
Vercel's git integration cannot connect to at all — so the documented
route amounted to "mirror to GitHub first", and nobody did.

vercel.json is gone, and with it the branch in next.config.ts that
switched off `output: "standalone"` when the VERCEL variable was
present. That branch was the only functional trace; everything else was
documentation and comments describing a second deployment path that did
not exist.

DEPLOYMENT.md loses its "two supported ways" framing and the whole
Vercel section — about fifty lines. Several statements next to it were
stale for a different reason and are corrected in the same pass: the
outbound-firewall table still listed Supabase's pooler (the database is
a container now, nothing leaves the server), the prerequisites still
demanded an existing Supabase project, and the .env table still asked
for a pooler connection string instead of the two new passwords.

The nightly job is described as what it is — a container in
docker-compose.yml — rather than as a replacement for Vercel Cron.

Migrations keep their references: two comments from July mention Vercel
Cron, and they describe what was true when they were written.

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

app/(app)/audit/page.tsx

@@ -21,8 +21,8 @@ function pageHref(params: SearchParams, page: number): string {
 }
 
 // Pinned to Vienna and built once: audit_log.occurred_at is a timestamptz, and
-// an unpinned formatter renders it in the *server's* zone — UTC in Docker and
-// on Vercel — so every entry would read an hour or two early for the people
+// an unpinned formatter renders it in the *server's* zone — UTC in the
+// container — so every entry would read an hour or two early for the people
 // the log is for.
 const dateTimeFormatter = new Intl.DateTimeFormat("de-AT", {
   day: "2-digit",