feat: add observability and Ant Design workbench

[PeterGuy326]

Summary

• rebuild the control room with Ant Design 6, responsive queue/DLQ workflows, and server-rendered component styles
• add canonical health probes, Prometheus metrics, runtime queue snapshots, and low-cardinality counters
• harden metrics authentication and the dashboard proxy boundary
• document local/Compose usage, scraping, alerting, and operational semantics

Validation

• backend: 73/73 tests with real Redis
• frontend: typecheck and production Webpack build
• Ant Design SSR: 217,707 bytes of extracted component CSS
• Docker Compose health/proxy/metrics smoke tests
• dependency audit, actionlint, compose config, and diff checks
• three focused P0/P1 reviews with no remaining findings

Closes #5