Adds health check endpoint

Adds a /health endpoint that checks MongoDB connectivity.
It returns a 200 OK status when healthy and a 503 Service
Unavailable status when unhealthy. Includes details about
the server's status, version, start time, and MongoDB's
status.  Also documents the new endpoint in the README.

Author: jerazost

README.md

@@ -92,6 +92,39 @@ curl -X POST https://your-quantux-backend.com/rest/user/token-exchange \
 
 The external API must implement `GET /v3/user/me` that returns a user object with at least `id` and `username` fields.
 
+## Health Check
+
+The server provides a health check endpoint at `/health` that checks MongoDB connectivity:
+
+- **Path**: `/health`
+- **Method**: `GET`
+- **Response**:
+  - `200 OK` when healthy (MongoDB is reachable)
+  - `503 Service Unavailable` when unhealthy (MongoDB is unreachable)
+
+The response includes:
+
+- `status`: "UP" or "DOWN"
+- `version`: Server version
+- `started`: Server start timestamp
+- `mongo.status`: "UP" or "DOWN"
+- `mongo.error`: Error message (only present when MongoDB is down)
+
+Example healthy response:
+
+```json
+{
+  "status": "UP",
+  "version": "4.5.6",
+  "started": "2024-01-01T12:00:00",
+  "mongo": {
+    "status": "UP"
+  }
+}
+```
+
+There is also a simpler status endpoint at `/rest/status.json` that returns version and start time without checking dependencies.
+
 ## Mongo optimization
 
 Start the `mongo` shell and run the following commands to set the correct mongo indexes

src/main/java/com/qux/MATC.java

@@ -66,6 +66,7 @@ public void start() {
 
     initTokenService(config);
     initStatus(router);
+    initHealth(router);
     initUserRest(config, router);
     initAppRest(router, config);
     initCommandRest(router);
@@ -117,6 +118,36 @@ private void initStatus(Router router) {
         .encodePrettily()));
   }
 
+  private void initHealth(Router router) {
+    router.route(HttpMethod.GET, "/health").handler(event -> {
+      JsonObject health = new JsonObject()
+          .put("status", "UP")
+          .put("version", VERSION)
+          .put("started", startedTime);
+
+      // Check MongoDB connectivity using a simple count operation
+      // Using an empty query on a collection to test connectivity
+      client.count("_health_check", new JsonObject(), ar -> {
+        if (ar.succeeded()) {
+          health.put("mongo", new JsonObject().put("status", "UP"));
+          event.response()
+              .setStatusCode(200)
+              .putHeader("Content-Type", "application/json")
+              .end(health.encodePrettily());
+        } else {
+          health.put("status", "DOWN");
+          health.put("mongo", new JsonObject()
+              .put("status", "DOWN")
+              .put("error", ar.cause() != null ? ar.cause().getMessage() : "Unknown error"));
+          event.response()
+              .setStatusCode(503)
+              .putHeader("Content-Type", "application/json")
+              .end(health.encodePrettily());
+        }
+      });
+    });
+  }
+
   private void initLibrary(Router router) {
 
     LibraryRest libs = new LibraryRest(this.tokenService, client);