feat: add E2E test suite with nightly CI, fix dashboard Dockerfile

E2E Tests:
- 17 integration tests covering: health, repos CRUD, findings lifecycle,
  cascade delete (SAST + DAST + pentest), DAST targets, stats overview
- TestServer harness: spins up agent API on random port with isolated
  MongoDB database per test, auto-cleanup
- Added lib.rs to expose agent internals for integration tests
- Nightly CI workflow with MongoDB service container (3 AM UTC)

Tests verify:
- Repository add/list/delete + duplicate rejection + invalid ID handling
- Finding creation, filtering by severity/repo, status updates, bulk updates
- Cascade delete: repo deletion removes all DAST targets, pentest sessions,
  attack chain nodes, DAST findings, SAST findings, and SBOM entries
- DAST target CRUD and empty finding list
- Stats overview accuracy with zero and populated data

Also:
- Fix Dockerfile.dashboard: bump dioxus-cli 0.7.3 → 0.7.4 (compile fix)
- Fix clippy: allow new_without_default for pattern scanners

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

compliance-agent/tests/integration/api/health.rs

@@ -0,0 +1,29 @@
+use crate::common::TestServer;
+
+#[tokio::test]
+async fn health_endpoint_returns_ok() {
+    let server = TestServer::start().await;
+
+    let resp = server.get("/api/v1/health").await;
+    assert_eq!(resp.status(), 200);
+
+    let body: serde_json::Value = resp.json().await.unwrap();
+    assert_eq!(body["status"], "ok");
+
+    server.cleanup().await;
+}
+
+#[tokio::test]
+async fn stats_overview_returns_zeroes_on_empty_db() {
+    let server = TestServer::start().await;
+
+    let resp = server.get("/api/v1/stats/overview").await;
+    assert_eq!(resp.status(), 200);
+
+    let body: serde_json::Value = resp.json().await.unwrap();
+    let data = &body["data"];
+    assert_eq!(data["repositories"], 0);
+    assert_eq!(data["total_findings"], 0);
+
+    server.cleanup().await;
+}