Add integration tests for Proof Chain and Reachability workflows

- Implement ProofChainTestFixture for PostgreSQL-backed integration tests.
- Create StellaOps.Integration.ProofChain project with necessary dependencies.
- Add ReachabilityIntegrationTests to validate call graph extraction and reachability analysis.
- Introduce ReachabilityTestFixture for managing corpus and fixture paths.
- Establish StellaOps.Integration.Reachability project with required references.
- Develop UnknownsWorkflowTests to cover the unknowns lifecycle: detection, ranking, escalation, and resolution.
- Create StellaOps.Integration.Unknowns project with dependencies for unknowns workflow.

tests/integration/StellaOps.Integration.ProofChain/ProofChainTestFixture.cs

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+// -----------------------------------------------------------------------------
+// ProofChainTestFixture.cs
+// Sprint: SPRINT_3500_0004_0003_integration_tests_corpus
+// Task: T1 - Proof Chain Integration Tests
+// Description: Test fixture for proof chain integration tests with PostgreSQL
+// -----------------------------------------------------------------------------
+
+using Microsoft.AspNetCore.Hosting;
+using Microsoft.AspNetCore.Mvc.Testing;
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Hosting;
+using Microsoft.Extensions.Logging;
+using Testcontainers.PostgreSql;
+
+namespace StellaOps.Integration.ProofChain;
+
+/// <summary>
+/// Test fixture for proof chain integration tests.
+/// Provides a fully configured Scanner WebService with PostgreSQL backing store.
+/// </summary>
+public sealed class ProofChainTestFixture : IAsyncLifetime
+{
+    private PostgreSqlContainer? _postgresContainer;
+    private WebApplicationFactory<Program>? _factory;
+    private bool _initialized;
+
+    /// <summary>
+    /// Initializes the test fixture, starting PostgreSQL container.
+    /// </summary>
+    public async Task InitializeAsync()
+    {
+        if (_initialized)
+            return;
+
+        // Start PostgreSQL container
+        _postgresContainer = new PostgreSqlBuilder()
+            .WithImage("postgres:16-alpine")
+            .WithDatabase("stellaops_test")
+            .WithUsername("test_user")
+            .WithPassword("test_password")
+            .WithPortBinding(5432, true)
+            .Build();
+
+        await _postgresContainer.StartAsync();
+
+        // Create the test web application factory
+        _factory = new WebApplicationFactory<Program>()
+            .WithWebHostBuilder(builder =>
+            {
+                builder.ConfigureAppConfiguration((context, config) =>
+                {
+                    // Override connection string with test container
+                    config.AddInMemoryCollection(new Dictionary<string, string?>
+                    {
+                        ["ConnectionStrings:ScannerDb"] = _postgresContainer.GetConnectionString(),
+                        ["Scanner:Authority:Enabled"] = "false",
+                        ["Scanner:AllowAnonymous"] = "true",
+                        ["Scanner:ProofChain:Enabled"] = "true",
+                        ["Scanner:ProofChain:SigningKeyId"] = "test-key",
+                        ["Scanner:ProofChain:AutoSign"] = "true",
+                        ["Logging:LogLevel:Default"] = "Warning"
+                    });
+                });
+
+                builder.ConfigureServices(services =>
+                {
+                    // Add test-specific service overrides if needed
+                    services.AddLogging(logging =>
+                    {
+                        logging.ClearProviders();
+                        logging.AddConsole();
+                        logging.SetMinimumLevel(LogLevel.Warning);
+                    });
+                });
+            });
+
+        _initialized = true;
+    }
+
+    /// <summary>
+    /// Creates an HTTP client for the test application.
+    /// </summary>
+    public async Task<HttpClient> CreateClientAsync()
+    {
+        if (!_initialized)
+        {
+            await InitializeAsync();
+        }
+
+        return _factory!.CreateClient(new WebApplicationFactoryClientOptions
+        {
+            AllowAutoRedirect = false
+        });
+    }
+
+    /// <summary>
+    /// Disposes of the test fixture resources.
+    /// </summary>
+    public async Task DisposeAsync()
+    {
+        _factory?.Dispose();
+
+        if (_postgresContainer is not null)
+        {
+            await _postgresContainer.DisposeAsync();
+        }
+    }
+}
+
+/// <summary>
+/// Placeholder for Program class detection.
+/// The actual Program class is from Scanner.WebService.
+/// </summary>
+#pragma warning disable CA1050 // Declare types in namespaces
+public partial class Program { }
+#pragma warning restore CA1050