feat: add notification channels with provider integrations and retry

Implement a channels system for notifying teams via external providers
(Discord, Slack, Teams, Google Chat, Telegram, Matrix, webhook) when
lifecycle events occur (capsule/template/host state changes).

- Channel CRUD API under /v1/channels (JWT-only auth)
- Test endpoint to verify config before saving (POST /v1/channels/test)
- Secret rotation endpoint (PUT /v1/channels/{id}/config)
- AES-256-GCM encryption for provider secrets (WRENN_ENCRYPTION_KEY)
- Redis stream event publishing from audit logger
- Background dispatcher with consumer group and retry (10s, 30s)
- Webhook delivery with HMAC-SHA256 signing (X-WRENN-SIGNATURE)
- shoutrrr integration for chat providers
- Secrets never exposed in API responses

.env.example

@@ -35,6 +35,10 @@ JWT_SECRET=
 WRENN_CA_CERT=
 WRENN_CA_KEY=
 
+# Channels (notification destinations)
+# AES-256-GCM key for encrypting channel secrets. Generate with: openssl rand -hex 32
+WRENN_ENCRYPTION_KEY=
+
 # OAuth
 OAUTH_GITHUB_CLIENT_ID=
 OAUTH_GITHUB_CLIENT_SECRET=