Add NATS/JetStream as an alternative broker to RabbitMQ

Add NATS/JetStream as an alternative broker to RabbitMQ

RabbitMQ's Erlang VM baseline costs ~300-500MB RES regardless of actual
traffic, which is wasteful for a small hobby-scale chatbot. Introduces a
runtime toggle (broker_backend = amqp | nats in local_config.ini) so a
deployment can opt into a NATS+JetStream backend instead, while defaulting
to amqp so existing deployments are unaffected.

- New common/broker.py abstraction (SyncBroker/AsyncBroker) with
  broker_amqp.py (thin, behavior-preserving wrapper around the existing
  pika calls) and broker_nats.py backends.
- Binding-key wildcard matching (some patterns, e.g. fun.py's mid-pattern
  "#", have no native NATS subject-algebra equivalent) is handled by
  subscribing broadly and filtering in-process via common/routing.py,
  lifted from the AMQP-routing simulator already used in integration tests.
- action_processing and plugin_registry go through JetStream for
  durability/late-subscriber buffering, matching today's AMQP guarantees.
- Ansible role: broker_backend/nats_uri toggle, optional local NATS server
  provisioning (nats_local), wait-for-broker.sh picked conditionally.

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