Add project files.

Gateway/Controllers/ExecutionController.cs

@@ -0,0 +1,25 @@
+using Gateway.Services;
+using Microsoft.AspNetCore.Mvc;
+using System.Text.Json;
+
+namespace Gateway.Controllers;
+
+[ApiController]
+[Route("api/execution")]
+public sealed class ExecutionController : ControllerBase
+{
+    private readonly ExecutionService _svc;
+    public ExecutionController(ExecutionService svc) => _svc = svc;
+
+    [HttpPost("request")]
+    public async Task<IActionResult> Execute([FromBody] JsonElement body)
+    {
+        if (body.ValueKind == JsonValueKind.Undefined || body.ValueKind == JsonValueKind.Null)
+            return BadRequest(new { ok = false, error = "Missing request body" });
+
+        var resp = await _svc.ExecuteAsync(body, HttpContext.RequestAborted);
+
+        // resp is JsonElement / JsonDocument / string json — you decide.
+        return Content(resp, "application/json");
+    }
+}