Add Security Center — multi-source event pipeline with dashboard UI

Introduces a unified security event store and HTTP API that ingests events
from any of the configured sources (API audit, Caddy access log, fail2ban,
shared_bans, future remote agents) and surfaces them in the dashboard.

New files:
  src/security/event-store.js      JSONL-backed store + in-memory query index
  src/security/host-registry.js    Registered hosts with per-host API keys
  src/security/event-workers.js    Tail-followers for Caddy/fail2ban/shared_bans logs
  routes/security.js               Events, hosts, ingest, SSE stream endpoints
  status/js/security-center.js     Dashboard modal with Overview/Events/Hosts tabs
  SECURITY-FEATURE.md              Full feature documentation
  DEAD-CODE.md, DUP-CODE.md, HARDENING.md   Prior audits

Modified:
  src/app.js                       Mount /api/v1/security/*
  src/utilities/middleware.js      Add ingest endpoints to PUBLIC_ROUTES
  src/security/audit-logger.js     Mirror audit events into security store
  server.js                        Start security workers on boot
  status/build.js                  Bundle security-center.js
  status/index.html                Add Security button to nav

status/index.html

@@ -205,6 +205,7 @@
             <button id="view-container-logs" aria-label="View container logs">📜 Container Logs</button>
             <button id="manage-notifications" aria-label="Manage notifications">🔔 Alerts</button>
             <button id="audit-log-btn" aria-label="Audit log">📜 Audit</button>
+            <button id="security-center-btn" aria-label="Security Center">🛡️ Security</button>
             <button id="docker-resources-btn" aria-label="Docker resources">🐳 Docker</button>
           </div>
         </div>