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>

docs/data-model.md

@@ -99,10 +99,9 @@ Ein einziges Rollenmodell, kein Mehrfach-Rollen-System:
 Funktion, deren Ausführungsrecht explizit auf `service_role` beschränkt ist
 (`revoke ... from public, anon, authenticated; grant ... to service_role`).
 Sie wird von `app/api/cron/apply-pending-changes/route.ts` aufgerufen —
-täglich per Vercel Cron (`vercel.json`), außerhalb von Vercel per
-Ersatz-Scheduler (siehe `DEPLOYMENT.md`, Docker-Cron-Sidecar). Die Route
-selbst authentifiziert per `CRON_SECRET`-Bearer-Token, nicht per
-Supabase-Session — es gibt keine anfragende Person, nur den Scheduler.
+täglich vom `cron`-Container aus `docker-compose.yml`. Die Route selbst
+authentifiziert per `CRON_SECRET`-Bearer-Token, nicht über eine Sitzung — es
+gibt keine anfragende Person, nur den Zeitplan.
 
 Idempotenz: `pending_org_changes.status` läuft `pending` → `applied` (oder
 `cancelled` bei Reorg-Undo); die Auswahl-Query filtert immer auf