`group_vars/matrix_servers` selects postgres whenever postgres is enabled, which
is the default, so postgres is what essentially every deployment runs. The
scenarios were testing sqlite - a path almost nobody is on.

How little that path is used is not a guess: the mautrix-meta bridges could not
start at all under sqlite, and nobody reported it. Testing the engine users are
actually on is worth more than keeping coverage of the one they are not, so no
scenario is left behind on sqlite.

Four of the eight scenarios have a database and are converted; the other four
have none and are untouched.

molecule-shared/tasks/postgres.yml stands Postgres up on the scenario's network,
with the data directory on a tmpfs since it is thrown away with the container.
The image is pinned at the major the postgres role deploys to new installations
and left to Renovate: when a new major lands, the PR bumping that pin runs every
scenario against it, which is the earliest warning we get that a component does
not cope.

Each scenario gives its database and user names that differ from the role's
defaults, so the component reaching the database proves the role built its
connection string out of them. The assertions moved from "a file appeared at the
path we configured" to "these tables exist", which is strictly stronger: tables
can only appear once the component has resolved the hostname, authenticated with
the credentials the role rendered, and run its migrations to completion.

Costs about 10 seconds per affected scenario (115s to 125s locally for
mautrix-whatsapp), on jobs that run in parallel.

Gotcha worth recording: since Postgres 18 the image puts PGDATA in a versioned
subdirectory and refuses to start if it finds a mount at the old
/var/lib/postgresql/data, so the tmpfs is mounted at /var/lib/postgresql.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SEH3vxYSQ5SV4N5z61eyGT

They were hard-wrapped at 80 characters, broke mid-parenthesis, and spent lines
restating what the code below them does.

Rewrapped at natural boundaries instead, with the narration dropped and only the
reasons, gotchas and surprises kept. Section dividers stay - they delineate long
plays rather than narrate them.

Comments only; no scenario behaviour changes.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SEH3vxYSQ5SV4N5z61eyGT

Three things that would not have scaled to 70 roles:

- The Python and Ansible dependency pins were about to be copied into every
  role. They now live once in molecule-shared/, which scenarios reference
  relatively, so they cannot drift apart.
- The helper container images used for probing were hardcoded inline. They
  are pinned once in molecule-shared/vars.yml, carry `# renovate:`
  annotations, and a custom manager in .github/renovate.json keeps them
  current - verified with a local Renovate dry run, which offers
  curl 8.11.1 -> 8.21.0 and python 3.13 -> 3.14-alpine. Seventy invisible
  hardcodes is the blindness class we have been removing elsewhere.
- Running a scenario meant knowing the venv and cd incantation. `just
  molecule <role>` does it, and with no argument lists the roles that have
  a scenario.

Molecule is deliberately not wired into prek: a run takes minutes, pulls
images and needs Docker, which is fine on request and not fine per commit.

docs/molecule-testing.md covers how to run and write these, including the
four things a role here needs that a standalone role does not. AGENTS.md
points at it rather than carrying the detail.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>