Test the Molecule scenarios against Postgres rather than sqlite

`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

docs/molecule-testing.md

@@ -91,6 +91,28 @@ own job there, so there is nothing to collide with.
 The directories are disposable; `var/` is gitignored. Delete `var/molecule-ansible-home/` to force
 a fresh install.
 
+## Databases
+
+Scenarios for roles that have a database run against **Postgres**, not sqlite.
+
+That is what `group_vars/matrix_servers` selects whenever postgres is enabled, which is the
+default, so it is what essentially every deployment runs. sqlite is a path almost nobody is on:
+a bug that stopped the mautrix-meta bridges from starting at all under sqlite sat unreported
+for a long time, which says plainly enough whose path is worth testing.
+
+`molecule-shared/tasks/postgres.yml` stands one up on the scenario's container network. Include
+it from `prepare.yml` and point the role at it with its own `_database_engine`, `_database_hostname`
+and credentials. Give the database and user names that differ from the role's defaults - then the
+component reaching the database at all proves the role built its connection string out of them.
+
+The image is pinned in `molecule-shared/vars.yml` at the major the postgres role deploys to new
+installations, and Renovate carries it forward. 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 with it.
+
+Prefer asserting on the schema the component created over a file on disk: tables can only appear
+once it has resolved the hostname, authenticated, and run its migrations.
+
 ## Reclaiming the disk space
 
 `just molecule-clean` removes what the runs leave under `var/`.