docs(vercel): lead with the four-step deploy, and fix the install e2e

[jouwdan]

Two things, bundled because the second is fallout from #244 and blocking CI.

The Vercel page predated the Deploy Button

It was not merely badly ordered — it was wrong. It told the reader to import their own repository and set a build command of:

community migrate && forum-web build

with no --at-root, so the artefact lands two directories from where Vercel reads it. And its "What you need" table asked for a managed PostgreSQL, a managed Redis, a mail provider and a domain. All four now arrive with the deploy.

The new shape

• ## Deploy it — four steps at the top. Press the button, connect the four products (Neon, Upstash, Blob, Resend), generate the two 32-character secrets, deploy, open /install. Plus the one value worth checking afterwards, APP_URL.
• ## Before you commit to this route — who it suits, what it costs, the limits no configuration moves.
• ## Things to know — the tick and its cadence, maxDuration, build-time migration, the installer, mail, blob vs bucket, both database strings, the derivation order.
• ## Configuring a project by hand — the manual route kept whole, with the build command corrected and the requirements table that used to sit at the top.
• ## Leaving Vercel — unchanged, and still the section that makes the rest of the page acceptable.

Nothing was deleted; the page went from 845 to ~920 lines. The reorganisation was done by parsing the file into sections and reassembling them, rather than retyping, so moved prose is byte-identical.

The two anchors other documents link into are preserved verbatim — #how-the-blob-store-authenticates (from disaster-recovery.md and scaling.md) and #when-a-derivation-cannot-resolve (from internationalisation.md). Self-links to the three renamed headings were rewritten; docs:links:check passes across 31 documents and 659 headings.

The install e2e assumed the installer migrates

install-no-js.spec.ts starts from what it called an empty database and expects to reach a validation error at "Create the administrator". #244 stopped the installer applying migrations, so it now fails at step 1 instead:

"Check the schema" did not finish. The database does not have every table this board needs…

The fixture had conflated two things under one seeded flag: applying the schema, and inserting fixture rows. They are separate now. The install database is { seeded: false, migrated: true } — schema, no rows — which is the state a board is genuinely in when someone opens /install: migrations run before the board serves anything, from the container entrypoint or the deploy build command.

A genuinely empty database would now fail that first step, and correctly so. That is the behaviour, not a gap in the test.

The board project's database is untouched: migrated defaults to seeded, so its code path is identical to before.

Verification

• pnpm verify green — 7,876 tests, 449 files
• the install project passes locally, watched go green after the fixture change
• the full suite was re-run locally with output captured; this box's pre-installed Chromium is older than the pinned Playwright, so it needs the config's existing PLAYWRIGHT_CHROMIUM_PATH hatch — CI has the matching browser and needs nothing
• docs:links:check, docs:index:check and site:docs:check all pass

Fixes MEI-135.

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