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_DEVOPS.md

@@ -86,6 +86,7 @@ At startup, services **self‑advertise** their semver & channel; the UI surface
 
 * **Primary**: `registry.stella-ops.org` (OCI v2, supports Referrers API).
 * **Mirrors**: GHCR (read‑only), regional mirrors for latency.
+  * Operational runbook: see `docs/ops/concelier-mirror-operations.md` for deployment profiles, CDN guidance, and sync automation.
 * **Pull by digest only** in Kubernetes/Compose manifests.
 
 **Gating policy**:
@@ -335,7 +336,8 @@ Prometheus + OTLP; Grafana dashboards ship in the charts.
 
 * **Vulnerability response**:
 
-  * Concelier red‑flag advisories trigger accelerated **stable** patch rollout; UI/CLI “security patch available” notice.
+  * Concelier red-flag advisories trigger accelerated **stable** patch rollout; UI/CLI “security patch available” notice.
+  * 2025-10: Pinned `MongoDB.Driver` **3.5.0** and `SharpCompress` **0.41.0** across services (DEVOPS-SEC-10-301) to eliminate NU1902/NU1903 warnings surfaced during scanner cache/worker test runs; future dependency bumps follow the same central override pattern.
 
 * **Backups/DR**: