feat: Shadow Run API Endpoint (Phase 4)

Implements FastEndpoints integration for 252+ trading-day validation trigger:

Contract-First Design (AGENTS.md v16.0):
- POST /api/shadow-runs (202 Accepted)
- Request: model_id, window_start, window_end, phase_filter
- Response: run_id, status, job_id, estimated_seconds
- Idempotency: Idempotency-Key header (deduplication)

Vertical Slice Components:
- Request.cs, Response.cs (DTOs with validation constraints)
- Validator.cs (FluentValidation): window >= 250 days, valid enum
- Handler.cs (Application): orchestrates command creation, Hangfire job enqueue
- Endpoint.cs (FastEndpoints): HTTP routing, error handling, 202 response
- Policy.cs: model existence validation (stub)

Integration:
- Hangfire background job client injection
- ShadowRunCommand creation with CorrelationId
- Queued to q-research (non-critical background queue)

Tests (9/9 passing):
- Validator: valid/invalid requests, phase filters, window constraints
- All validation scenarios: empty model, short window, invalid phase

Architecture Adherence:
- SOLID: Endpoint → Handler → Validator → Policy separation
- Complexity: Each component cyclomatic < 10
- Safety: Idempotent request (client-supplied key), async job model (202 response)
- Maturity: Contract verified, tests before implementation

Next Phase (Pending):
- Hangfire Job registration in Program.cs
- GET /api/shadow-runs/{run_id} polling endpoint
- E2E test: trigger → job execution → result persistence

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/KArtSell.Host/Features/ShadowRun/Endpoint.cs

@@ -0,0 +1,63 @@
+using FastEndpoints;
+using KArtSell.BuildingBlocks.Time;
+using Microsoft.Extensions.Logging;
+
+namespace KArtSell.Host.Features.ShadowRun;
+
+/// <summary>
+/// POST /api/shadow-runs
+/// Initiate a 252+ trading-day model validation run.
+/// Returns 202 Accepted with job tracking info.
+/// </summary>
+public sealed class InitiateShadowRunEndpoint : Endpoint<InitiateShadowRunRequest, InitiateShadowRunResponse>
+{
+    private InitiateShadowRunHandler? _handler;
+    private ILogger<InitiateShadowRunEndpoint>? _logger;
+
+    private static readonly Action<ILogger, Guid, Exception?> LogRequestReceived =
+        LoggerMessage.Define<Guid>(
+            LogLevel.Information,
+            new EventId(1, nameof(LogRequestReceived)),
+            "Shadow run request received: {ModelId}");
+
+    private static readonly Action<ILogger, Exception?> LogRequestFailed =
+        LoggerMessage.Define(
+            LogLevel.Error,
+            new EventId(2, nameof(LogRequestFailed)),
+            "Shadow run initiation failed");
+
+    public override void Configure()
+    {
+        Post("/api/shadow-runs");
+        AllowAnonymous();  // TODO: Add RBAC (researcher role required)
+        Validator<InitiateShadowRunValidator>();
+    }
+
+    public override async Task HandleAsync(InitiateShadowRunRequest req, CancellationToken ct)
+    {
+        _handler = Resolve<InitiateShadowRunHandler>();
+        _logger = Resolve<ILogger<InitiateShadowRunEndpoint>>();
+
+        var correlationId = HttpContext.Items["CorrelationId"] as Guid? ?? Guid.NewGuid();
+
+        try
+        {
+            LogRequestReceived(_logger, req.ModelId, null);
+
+            var response = await _handler.HandleAsync(req, correlationId, ct);
+
+            // 202 Accepted: Job queued, results available later via polling
+            HttpContext.Response.StatusCode = 202;
+            await HttpContext.Response.WriteAsJsonAsync(response, ct);
+        }
+        catch (InvalidOperationException ex)
+        {
+            ThrowError(ex.Message, 400);
+        }
+        catch (Exception ex)
+        {
+            LogRequestFailed(_logger, ex);
+            ThrowError("Shadow run initiation failed. Please retry.", 500);
+        }
+    }
+}