Add JaCoCo for test coverage and update Drone CI settings

Integrate JaCoCo to enforce and report test coverage with a minimum threshold of 65%. Update Drone CI configuration to use a new Docker registry (rubble.se:5000) and add support for authentication with secrets.

build.gradle

@@ -2,6 +2,7 @@ plugins {
     id 'java'
     id 'org.springframework.boot' version '3.3.4'
     id 'io.spring.dependency-management' version '1.1.6'
+    id 'jacoco'
 }
 
 group = 'se.urmo'
@@ -21,4 +22,31 @@ dependencies {
     testImplementation 'org.springframework.boot:spring-boot-starter-test'
 }
 
+test {
+    useJUnitPlatform()
+    finalizedBy jacocoTestReport
+}
+
+jacocoTestReport {
+    dependsOn test
+    reports {
+        xml.required = true
+        html.required = true
+    }
+}
+
+jacocoTestCoverageVerification {
+    violationRules {
+        rule {
+            limit {
+                counter = 'LINE'
+                value = 'COVEREDRATIO'
+                minimum = 0.65 // börja rimligt (65%), höj senare
+            }
+        }
+    }
+}
+
+check.dependsOn jacocoTestCoverageVerification
+
 tasks.test { useJUnitPlatform() }