Add support for Meowlnir

Meowlnir (https://github.com/maunium/meowlnir) is a Matrix moderation
bot which speaks the same policy-list protocol as Mjolnir and Draupnir,
but runs as an appservice and can override individual policies coming
from ban lists you do not control.

Bots and their management rooms live only in Meowlnir's own database —
nothing in its configuration file can declare one — so the role
provisions them through the management API from a declarative roster
(matrix_bot_meowlnir_bots_custom), applied under the
ensure-matrix-users-created tag. Management rooms may be declared or
created for you; bots and rooms no longer declared get pruned.

Wrapper scripts for driving the management API by hand are installed
to /matrix/meowlnir/bin.

Meowlnir re-runs its configuration upgrader in memory on every start,
so a literal `generate` value yields a new secret per restart. All
secrets are therefore rendered explicitly, validation rejects
`generate`, and the configuration directory is mounted read-only.

Draupnir and Meowlnir both want synapse-http-antispam, which the
playbook wires up to a single consumer. The wiring prefers Draupnir,
and both roles fail the run when each claims it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/configuring-playbook.md

@@ -208,6 +208,8 @@ Bots provide various additional functionality to your installation.
 
 - [Setting up Draupnir for all/D4A](configuring-playbook-appservice-draupnir-for-all.md) — like the [Draupnir bot](configuring-playbook-bot-draupnir.md) mentioned above, but running in appservice mode and supporting multiple instances
 
+- [Setting up Meowlnir](configuring-playbook-bot-meowlnir.md) — a moderation tool/bot which speaks the same policy list protocol as Draupnir and Mjolnir, but runs in appservice mode and can override policies coming from lists you do not control
+
 - [Setting up Buscarron](configuring-playbook-bot-buscarron.md) — a bot you can use to send any form (HTTP POST, HTML) to a (encrypted) Matrix room
 
 ### Administration