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.

src/StellaOps.Cli/Commands/CommandFactory.cs

@@ -358,6 +358,48 @@ internal static class CommandFactory
             return CommandHandlers.HandleExcititorExportAsync(services, format, delta, scope, since, provider, output, verbose, cancellationToken);
         });
 
+        var backfill = new Command("backfill-statements", "Replay historical raw documents into Excititor statements.");
+        var backfillRetrievedSinceOption = new Option<DateTimeOffset?>("--retrieved-since")
+        {
+            Description = "Only process raw documents retrieved on or after the provided ISO-8601 timestamp."
+        };
+        var backfillForceOption = new Option<bool>("--force")
+        {
+            Description = "Reprocess documents even if statements already exist."
+        };
+        var backfillBatchSizeOption = new Option<int>("--batch-size")
+        {
+            Description = "Number of raw documents to fetch per batch (default 100)."
+        };
+        var backfillMaxDocumentsOption = new Option<int?>("--max-documents")
+        {
+            Description = "Optional maximum number of raw documents to process."
+        };
+        backfill.Add(backfillRetrievedSinceOption);
+        backfill.Add(backfillForceOption);
+        backfill.Add(backfillBatchSizeOption);
+        backfill.Add(backfillMaxDocumentsOption);
+        backfill.SetAction((parseResult, _) =>
+        {
+            var retrievedSince = parseResult.GetValue(backfillRetrievedSinceOption);
+            var force = parseResult.GetValue(backfillForceOption);
+            var batchSize = parseResult.GetValue(backfillBatchSizeOption);
+            if (batchSize <= 0)
+            {
+                batchSize = 100;
+            }
+            var maxDocuments = parseResult.GetValue(backfillMaxDocumentsOption);
+            var verbose = parseResult.GetValue(verboseOption);
+            return CommandHandlers.HandleExcititorBackfillStatementsAsync(
+                services,
+                retrievedSince,
+                force,
+                batchSize,
+                maxDocuments,
+                verbose,
+                cancellationToken);
+        });
+
         var verify = new Command("verify", "Verify Excititor exports or attestations.");
         var exportIdOption = new Option<string?>("--export-id")
         {
@@ -408,6 +450,7 @@ internal static class CommandFactory
         excititor.Add(resume);
         excititor.Add(list);
         excititor.Add(export);
+        excititor.Add(backfill);
         excititor.Add(verify);
         excititor.Add(reconcile);
         return excititor;