feat(graph): add Postgres graph runtime repository + compatibility endpoints

Introduces IGraphRuntimeRepository + PostgresGraphRuntimeRepository that back
runtime-path graph reads with real persistence. Graph.Api Program.cs wires
the new repository into the DI graph. InMemory* services get small cleanups
so they remain viable for tests and local dev.

CompatibilityEndpoints: extends the integration-test surface.

Tests: GraphPostgresRuntimeIntegrationTests,
GraphRuntimeRepositoryRegistrationTests, expanded
GraphCompatibilityEndpointsIntegrationTests.

Docs: graph architecture page updated.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/Graph/__Tests/StellaOps.Graph.Api.Tests/GraphRuntimeRepositoryRegistrationTests.cs

@@ -0,0 +1,63 @@
+using Microsoft.AspNetCore.Builder;
+using Microsoft.AspNetCore.Hosting;
+using Microsoft.AspNetCore.Mvc.Testing;
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.DependencyInjection;
+using StellaOps.Graph.Api.Services;
+
+namespace StellaOps.Graph.Api.Tests;
+
+public sealed class GraphRuntimeRepositoryRegistrationTests : IClassFixture<GraphApiPostgresFixture>
+{
+    private readonly GraphApiPostgresFixture _fixture;
+
+    public GraphRuntimeRepositoryRegistrationTests(GraphApiPostgresFixture fixture)
+    {
+        _fixture = fixture;
+    }
+
+    [Fact]
+    [Trait("Category", "Integration")]
+    [Trait("Intent", "Registration")]
+    public void Resolves_PostgresRuntimeRepository_WhenConnectionStringIsConfigured()
+    {
+        using var factory = CreateFactory(_fixture.ConnectionString);
+        using var scope = factory.Services.CreateScope();
+
+        var repository = scope.ServiceProvider.GetRequiredService<IGraphRuntimeRepository>();
+
+        Assert.IsType<PostgresGraphRuntimeRepository>(repository);
+    }
+
+    [Fact]
+    [Trait("Category", "Integration")]
+    [Trait("Intent", "Registration")]
+    public void Resolves_EmptyInMemoryRuntimeRepository_WhenConnectionStringIsMissing()
+    {
+        using var factory = CreateFactory(string.Empty);
+        using var scope = factory.Services.CreateScope();
+
+        var repository = scope.ServiceProvider.GetRequiredService<IGraphRuntimeRepository>();
+        var inMemoryRepository = Assert.IsType<InMemoryGraphRepository>(repository);
+
+        Assert.Empty(inMemoryRepository.GetCompatibilityNodes("acme"));
+        Assert.Empty(inMemoryRepository.GetCompatibilityEdges("acme"));
+    }
+
+    private static WebApplicationFactory<Program> CreateFactory(string? connectionString)
+    {
+        return new WebApplicationFactory<Program>()
+            .WithWebHostBuilder(builder =>
+            {
+                builder.UseEnvironment("Development");
+                builder.ConfigureAppConfiguration((_, configurationBuilder) =>
+                {
+                    configurationBuilder.AddInMemoryCollection(new Dictionary<string, string?>
+                    {
+                        ["Postgres:Graph:ConnectionString"] = connectionString,
+                        ["Postgres:Graph:SchemaName"] = "graph",
+                    });
+                });
+            });
+    }
+}