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>

lib/format.ts

@@ -1,7 +1,7 @@
 // This app stores dates as date-only strings ("YYYY-MM-DD") and timestamps as
 // timestamptz, and is used from a single timezone. Every conversion here is
 // pinned to Europe/Vienna rather than the runtime's zone: the server renders
-// in UTC (Docker/Vercel) while the browser renders in Vienna, so an unpinned
+// in UTC (im Container) while the browser renders in Vienna, so an unpinned
 // formatter produces a different day on each side — wrong dates for the user
 // near midnight, and a React hydration mismatch.
 const TIMEZONE = "Europe/Vienna";