chore: add JaCoCo coverage enforcement to backend

- Add jacoco plugin (bundled with Gradle, no extra dependency)
- jacocoTestReport: generates HTML + XML reports, runs after test
- jacocoTestCoverageVerification: enforces 70% line coverage and
  60% branch coverage at the bundle level
- Wire jacocoTestCoverageVerification into tasks.named('check') so
  ./gradlew check blocks if coverage drops below thresholds
- HTML report output: backend/build/reports/jacoco/index.html
- test task finalizedBy jacocoTestReport so report is always
  available after running tests

backend/build.gradle

@@ -1,5 +1,6 @@
 plugins {
 	id 'java'
+	id 'jacoco'
 	id 'org.springframework.boot' version '4.0.6'
 	id 'io.spring.dependency-management' version '1.1.7'
 }
@@ -44,4 +45,39 @@ dependencies {
 
 tasks.named('test') {
 	useJUnitPlatform()
+	finalizedBy jacocoTestReport
+}
+
+jacoco {
+	toolVersion = "0.8.12"
+}
+
+jacocoTestReport {
+	dependsOn test
+	reports {
+		xml.required = true
+		csv.required = false
+		html.required = true
+	}
+}
+
+jacocoTestCoverageVerification {
+	dependsOn jacocoTestReport
+	violationRules {
+		rule {
+			limit {
+				minimum = 0.70
+			}
+		}
+		rule {
+			limit {
+				counter = 'BRANCH'
+				minimum = 0.60
+			}
+		}
+	}
+}
+
+tasks.named('check').configure {
+	dependsOn jacocoTestCoverageVerification
 }