feat: upstream auth for all protocols (Docker, Maven, npm, PyPI)

Wire up basic auth credentials for upstream registry proxying:
- Docker: pass configured auth to Bearer token requests
- Maven: support url|auth format in NORA_MAVEN_PROXIES env var
- npm: add NORA_NPM_PROXY_AUTH env var
- PyPI: add NORA_PYPI_PROXY_AUTH env var
- Mask credentials in logs (never log plaintext passwords)

Config examples:
  NORA_DOCKER_UPSTREAMS="https://registry.corp.com|user:pass"
  NORA_MAVEN_PROXIES="https://nexus.corp.com/maven2|user:pass"
  NORA_NPM_PROXY_AUTH="user:pass"
  NORA_PYPI_PROXY_AUTH="user:pass"

nora-registry/src/ui/api.rs

@@ -205,7 +205,12 @@ pub async fn api_dashboard(State(state): State<Arc<AppState>>) -> Json<Dashboard
         MountPoint {
             registry: "Maven".to_string(),
             mount_path: "/maven2/".to_string(),
-            proxy_upstream: state.config.maven.proxies.first().cloned(),
+            proxy_upstream: state
+                .config
+                .maven
+                .proxies
+                .first()
+                .map(|p| p.url().to_string()),
         },
         MountPoint {
             registry: "npm".to_string(),