core: Implement /health endpoint (healthcheck). Resolves #12784 (#12798)

src/Jackett.Server/Controllers/HealthcheckController.cs

@@ -0,0 +1,23 @@
+using System.Threading.Tasks;
+using Microsoft.AspNetCore.Authorization;
+using Microsoft.AspNetCore.Mvc;
+using Newtonsoft.Json.Linq;
+
+namespace Jackett.Server.Controllers
+{
+    [AllowAnonymous]
+    [Route("health")]
+    public class HealthcheckController : Controller
+    {
+        [HttpGet]
+        [HttpHead]
+        public Task<IActionResult> Health()
+        {
+            var jsonReply = new JObject
+            {
+                ["status"] = "OK"
+            };
+            return Task.FromResult<IActionResult>(Json(jsonReply));
+        }
+    }
+}