`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

Both roles derived `..._appservice_database_uri` as `'sqlite:///' + <path>`.
mautrix-go hands that string to go-sqlite3 as a filename rather than parsing it
as a URL, so the bridge cannot open its database and dies at startup:

  FTL Failed to initialize database
      error="... unable to open database file: no such file or directory"

Every other mautrix bridge role here passes the bare in-container path.

This has stayed hidden because group_vars/matrix_servers selects postgres
whenever postgres is enabled, which is the default - so almost nobody reaches
the sqlite branch. Anyone who does gets a bridge that never starts.

Found by the mautrix-meta-messenger Molecule scenario, which runs sqlite
deliberately. The scenario's override is dropped and its assertion now compares
the rendered URI against the path the role defines, so the derived value is
what is under test rather than the scenario's own.

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

Stands the bridge up against the shared homeserver stub and checks that it
starts on the configuration the role rendered: the unit is active with no
automatic restarts, the appservice listener on 29319 answers, and the bridge
created its sqlite database under the role's data path as the role's uid.
Nothing is bridged - there is no Facebook or Messenger account involved and
there is deliberately never going to be one.

The thing worth proving for this role in particular is the Meta mode. One
upstream codebase serves several Meta networks, and
`matrix_bridge_mautrix_meta_messenger_meta_mode` is what picks which one; the
role expands it into an appservice id, a ghost username prefix, a bot
displayname and the bridge's `tor` switch. The scenario therefore runs in
`facebook-tor` mode rather than the role's default `messenger`, and asserts on
all four - against the parsed configuration, not substrings. The registration
namespaces are checked by what they match rather than by comparing regexes:
this mode's ghosts are covered, the default mode's are not.

Since v26.07 the Instagram bridge is published to the same image repository
with an `ig-` tag prefix, so the running image is compared against the whole
tag `defaults/main.yml` pins rather than by substring - an `ig-` prefix would
mean this role pulled the other bridge's image.

Surprising: the role's derived sqlite URI does not work. For `sqlite3-fk-wal`
it builds `sqlite:///` + the in-container path, and the bridge hands that to
go-sqlite3 as a plain filename rather than parsing it as a URL, so it dies at
startup with `unable to open database file: no such file or directory`. Every
other mautrix role here uses a bare path. The playbook selects postgres
whenever postgres is enabled, which is the default, so nothing normally reaches
that code path. The scenario overrides the URI rather than changing the role's
default, which is consumer-visible; matrix-bridge-mautrix-meta-instagram has
the same expression.

Falsified by setting `matrix_bridge_mautrix_meta_messenger_meta_mode` back to
`messenger` and re-running: the run failed at "Assert the configuration
reflects the Meta mode the scenario selected", on the
`appservice.id == 'facebook-tor'` clause.

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