Proves the bridge starts on the configuration this role rendered: the
systemd unit is active and has not been restarting, the appservice port is
open, config.yaml and registration.yaml (parsed, not grepped) carry the
scenario's homeserver address, bot username, tokens, command prefix,
avatar-proxy key, public address and log level, the sqlite database was
created under the role's data path as the role's uid, and the running
container is the version defaults/main.yml pins.

It does not bridge anything. Discord is behind an account, which is where a
scenario stops being a test of this repository.

Two things are specific to this role rather than copied from the
mautrix-whatsapp scenario:

- mautrix-discord is the only bridge here whose validate_config.yml requires
  a public address. It is composed from hostname + scheme + path prefix, and
  those same three feed the Traefik avatar-proxy labels, so the scenario
  picks a non-`/` path prefix and a non-default scheme and asserts the
  composed router rule and strip-prefix middleware - both in the label file
  and, read back off the running container, as labels Docker accepted.
  Traefik labels are left enabled here for that reason, unlike in the
  mautrix-whatsapp scenario which asserts their absence.

- matrix_bridge_mautrix_discord_bridge_double_puppet_server_map_default in
  the role's defaults references matrix_bridge_beeper_linkedin_homeserver_domain
  and matrix_bridge_beeper_linkedin_homeserver_address - variables belonging
  to a different role, evidently copy-pasted from
  matrix-bridge-beeper-linkedin, and unique to this role among the bridges.
  A playbook run has every role's defaults in scope so it resolves silently;
  a role scenario has only this role loaded and the template fails on the
  undefined name. The scenario neutralises it in its own group_vars rather
  than touching the role.

Falsified the crash-loop assertion: pointing the in-container sqlite path at
a directory that does not exist makes the bridge exit on startup, and the run
then fails at "Assert the service is active and has not been restarting" with
"activating after 5 automatic restart(s)". Reverted, and green again since -
including idempotence.