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

go.mod

@@ -4,6 +4,7 @@ go 1.25.8
 
 require (
 	connectrpc.com/connect v1.19.1
+	github.com/containrrr/shoutrrr v0.8.0
 	github.com/go-chi/chi/v5 v5.2.5
 	github.com/golang-jwt/jwt/v5 v5.3.1
 	github.com/google/uuid v1.6.0
@@ -22,9 +23,12 @@ require (
 require (
 	github.com/cespare/xxhash/v2 v2.3.0 // indirect
 	github.com/dgryski/go-rendezvous v0.0.0-20200823014737-9f7001d12a5f // indirect
+	github.com/fatih/color v1.15.0 // indirect
 	github.com/jackc/pgpassfile v1.0.0 // indirect
 	github.com/jackc/pgservicefile v0.0.0-20240606120523-5a60cdf6a761 // indirect
 	github.com/jackc/puddle/v2 v2.2.2 // indirect
+	github.com/mattn/go-colorable v0.1.13 // indirect
+	github.com/mattn/go-isatty v0.0.17 // indirect
 	go.uber.org/atomic v1.11.0 // indirect
 	golang.org/x/sync v0.20.0 // indirect
 	golang.org/x/text v0.35.0 // indirect