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/dev/EXCITITOR_STATEMENT_BACKFILL.md

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+# Excititor Statement Backfill Runbook
+
+Last updated: 2025-10-19
+
+## Overview
+
+Use this runbook when you need to rebuild the `vex.statements` collection from historical raw documents. Typical scenarios:
+
+- Upgrading the statement schema (e.g., adding severity/KEV/EPSS signals).
+- Recovering from a partial ingest outage where statements were never persisted.
+- Seeding a freshly provisioned Excititor deployment from an existing raw archive.
+
+Backfill operates server-side via the Excititor WebService and reuses the same pipeline that powers the `/excititor/statements` ingestion endpoint. Each raw document is normalized, signed metadata is preserved, and duplicate statements are skipped unless the run is forced.
+
+## Prerequisites
+
+1. **Connectivity to Excititor WebService** – the CLI uses the backend URL configured in `stellaops.yml` or the `--backend-url` argument.
+2. **Authority credentials** – the CLI honours the existing Authority client configuration; ensure the caller has permission to invoke admin endpoints.
+3. **Mongo replica set** (recommended) – causal consistency guarantees rely on majority read/write concerns. Standalone deployment works but skips cross-document transactions.
+
+## CLI command
+
+```
+stellaops excititor backfill-statements \
+  [--retrieved-since <ISO8601>] \
+  [--force] \
+  [--batch-size <int>] \
+  [--max-documents <int>]
+```
+
+| Option | Description |
+| ------ | ----------- |
+| `--retrieved-since` | Only process raw documents fetched on or after the specified timestamp (UTC by default). |
+| `--force` | Reprocess documents even if matching statements already exist (useful after schema upgrades). |
+| `--batch-size` | Number of raw documents pulled per batch (default `100`). |
+| `--max-documents` | Optional hard limit on the number of raw documents to evaluate. |
+
+Example – replay the last 48 hours of Red Hat ingest while keeping existing statements:
+
+```
+stellaops excititor backfill-statements \
+  --retrieved-since "$(date -u -d '48 hours ago' +%Y-%m-%dT%H:%M:%SZ)"
+```
+
+Example – full replay with forced overwrites, capped at 2,000 documents:
+
+```
+stellaops excititor backfill-statements --force --max-documents 2000
+```
+
+The command returns a summary similar to:
+
+```
+Backfill completed: evaluated 450, backfilled 180, claims written 320, skipped 270, failures 0.
+```
+
+## Behaviour
+
+- Raw documents are streamed in ascending `retrievedAt` order.
+- Each document is normalized using the registered VEX normalizers (CSAF, CycloneDX, OpenVEX).
+- Statements are appended through the same `IVexClaimStore.AppendAsync` path that powers `/excititor/statements`.
+- Duplicate detection compares `Document.Digest`; duplicates are skipped unless `--force` is specified.
+- Failures are logged with the offending digest and continue with the next document.
+
+## Observability
+
+- CLI logs aggregate counts and the backend logs per-digest warnings or errors.
+- Mongo writes carry majority write concern; expect backfill throughput to match ingest baselines (≈5 seconds warm, 30 seconds cold).
+- Monitor the `excititor.storage.backfill` log scope for detailed telemetry.
+
+## Post-run verification
+
+1. Inspect the `vex.statements` collection for the targeted window (check `InsertedAt`).
+2. Re-run the Excititor storage test suite if possible:
+   ```
+   dotnet test src/StellaOps.Excititor.Storage.Mongo.Tests/StellaOps.Excititor.Storage.Mongo.Tests.csproj
+   ```
+3. Optionally, call `/excititor/statements/{vulnerabilityId}/{productKey}` to confirm the expected statements exist.
+
+## Rollback
+
+If a forced run produced incorrect statements, use the standard Mongo rollback procedure:
+
+1. Identify the `InsertedAt` window for the backfill run.
+2. Delete affected records from `vex.statements` (and any downstream exports if applicable).
+3. Rerun the backfill command with corrected parameters.