Add channel test providers for Email, Slack, Teams, and Webhook

- Implemented EmailChannelTestProvider to generate email preview payloads.
- Implemented SlackChannelTestProvider to create Slack message previews.
- Implemented TeamsChannelTestProvider for generating Teams Adaptive Card previews.
- Implemented WebhookChannelTestProvider to create webhook payloads.
- Added INotifyChannelTestProvider interface for channel-specific preview generation.
- Created ChannelTestPreviewContracts for request and response models.
- Developed NotifyChannelTestService to handle test send requests and generate previews.
- Added rate limit policies for test sends and delivery history.
- Implemented unit tests for service registration and binding.
- Updated project files to include necessary dependencies and configurations.

docs/ARCHITECTURE_AUTHORITY.md

@@ -229,6 +229,8 @@ GET  /admin/metrics                 # Prometheus exposition (token issue rates,
 GET  /admin/healthz|readyz          # health/readiness
 ```
 
+Declared client `audiences` flow through to the issued JWT `aud` claim and the token request's `resource` indicators. Authority relies on this metadata to enforce DPoP nonce challenges for `signer`, `attestor`, and other high-value services without requiring clients to repeat the audience parameter on every request.
+
 ---
 
 ## 11) Integration hard lines (what resource servers must enforce)
@@ -286,6 +288,8 @@ authority:
     nonce:
       enable: true
       ttlSeconds: 600
+      store: redis
+      redisConnectionString: "redis://authority-redis:6379?ssl=false"
   mtls:
     enable: true
     caBundleFile: /etc/ssl/mtls/clients-ca.pem