feat: NCS-37 add backcore Quarkus module skeleton

Introduces the backcore Quarkus REST module with build configuration,
application.yml, and a smoke test confirming Quarkus boots successfully.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

modules/backcore/src/main/resources/application.yml

@@ -0,0 +1,3 @@
+quarkus:
+  http:
+    port: 8080

modules/backcore/src/test/scala/de/nowchess/backcore/BackcoreStartupTest.scala

@@ -0,0 +1,11 @@
+package de.nowchess.backcore
+
+import io.quarkus.test.junit.QuarkusTest
+import org.junit.jupiter.api.Test
+
+@QuarkusTest
+class BackcoreStartupTest:
+  @Test
+  def applicationStarts(): Unit =
+    // If we get here the Quarkus container started successfully
+    ()