Restructure solution layout by module

src/Signals/StellaOps.Signals/Models/CallgraphIngestRequest.cs

@@ -0,0 +1,16 @@
+using System.Collections.Generic;
+using System.ComponentModel.DataAnnotations;
+
+namespace StellaOps.Signals.Models;
+
+/// <summary>
+/// API request payload for callgraph ingestion.
+/// </summary>
+public sealed record CallgraphIngestRequest(
+    [property: Required] string Language,
+    [property: Required] string Component,
+    [property: Required] string Version,
+    [property: Required] string ArtifactContentType,
+    [property: Required] string ArtifactFileName,
+    [property: Required] string ArtifactContentBase64,
+    IReadOnlyDictionary<string, string?>? Metadata);