Wire backend tests and coverage into ./gradlew check.

Root check only ran frontend lint, unit tests, and E2E, so backend JUnit
and JaCoCo gates were skipped despite AGENTS.md documenting otherwise.

- Make check depend on :backend:check and frontendE2E as siblings
- Update AGENTS.md comment to match the real task order

AGENTS.md

@@ -37,7 +37,7 @@ docker compose up -d    # starts postgres, backend, frontend
 ### All-in-one
 
 ```bash
-./gradlew check         # frontend lint → frontend test → backend test → coverage verification
+./gradlew check         # frontend lint → frontend test → backend test+coverage → E2E (Docker)
 ./gradlew coverage      # backend + frontend tests with coverage reports
 ./gradlew up            # docker compose up -d
 ./gradlew down          # docker compose down

build.gradle

@@ -44,7 +44,8 @@ tasks.register('frontendE2E', Exec) {
 }
 
 tasks.named('check').configure {
-    dependsOn frontendLint, frontendTest, frontendE2E
+    description = 'Full verification: frontend lint/tests, backend tests+coverage, E2E'
+    dependsOn ':backend:check', frontendE2E
 }
 
 tasks.register('up', Exec) {