Summary

Implement a generic mechanism for services to subscribe to #settings message bus channel and react to runtime setting changes by rebuilding affected service instances (ProviderRegistry, ModelResolver, MemoryBackend, etc.).

Context

SettingsService already publishes change notifications to the #settings message bus channel when settings are written or deleted (see service.py:_publish_change()). However, no services currently subscribe to these notifications. Startup-time consumers build service instances once from config and never update:

• ProviderRegistry.from_config(root_config.providers) — builds driver instances
• ModelResolver.from_config(root_config.providers) — builds model index
• ModelRouter(routing_config, providers) — builds routing strategy
• create_memory_backend(config.memory) — builds memory backend

When a setting changes at runtime (e.g., provider API key rotated via the settings API), these services continue using stale config until restart.

Scope

• Create a SettingsSubscriber protocol or base class that services implement
• Implement a listener on the #settings message bus channel that dispatches to registered subscribers
• Each subscriber declares which (namespace, key) pairs it cares about
• On change: subscriber rebuilds the affected service and swaps it into AppState
• Thread-safe service swap (atomic reference replacement)
• Graceful handling of rebuild failures (log error, keep old instance)

Design Considerations

• Memory backend has restart_required=True — change notification should flag this to the operator rather than attempting hot-reload
• Provider services could be rebuilt without restart
• Coordination settings take effect per-request already (via ConfigResolver composed reads from feat: migrate existing config paths to read through SettingsService #497), so no subscriber needed there

Related

• feat: migrate existing config paths to read through SettingsService #497 (creates ConfigResolver, migrates controller-layer reads)
• feat: refactor structural data endpoints to read from SettingsService via JSON settings #502 (structural data JSON settings)
• feat: implement provider management (multi-auth, runtime CRUD, local providers) #451 (provider runtime CRUD — depends on this for hot-reload after provider add/edit/remove)