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>

.env.example

@@ -57,7 +57,9 @@ AUTH_MICROSOFT_ENTRA_ID_ISSUER=https://login.microsoftonline.com/<verzeichnis-id
 # Auth.js die Rückruf-Adresse aus den Request-Headern.
 AUTH_URL=
 
-# Shared secret Vercel Cron sends as `Authorization: Bearer <value>` when it
-# calls /api/cron/apply-pending-changes (set the same value in the Vercel
-# project's env vars). Generate with e.g. `openssl rand -hex 32`.
+# Gemeinsames Geheimnis, mit dem sich der nächtliche Lauf ausweist: der
+# `cron`-Container schickt es als `Authorization: Bearer <wert>` an
+# /api/cron/apply-pending-changes. Ohne angemeldete Person gibt es keine
+# Sitzung, an der die Route den Aufruf prüfen könnte.
+# Erzeugen mit: openssl rand -hex 32
 CRON_SECRET=