WP-40/41/42: dashboard, security hardening, deployment and operations docs

Dashboard endpoint and page aggregating inventory, vulnerabilities, cluster
health, backups and recent jobs. Security headers (CSP, nosniff, DENY,
referrer policy), 1 MB body limit, configurable login rate limit, audit
steps in CI. Installer script, systemd unit, install/architecture docs.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

backend/crates/api/src/openapi.rs

@@ -34,6 +34,7 @@ impl Modify for BearerAuth {
         crate::backups::delete_target, crate::backups::test_target, crate::backups::list_strategies, crate::backups::get_strategy,
         crate::backups::create_strategy, crate::backups::update_strategy, crate::backups::delete_strategy,
         crate::backups::run_strategy, crate::backups::records,
+        crate::dashboard::dashboard,
     ),
     modifiers(&BearerAuth)
 )]