feat(server): add local demo data mode

[ZingerLittleBee]

Summary

• add a guarded local demo data mode that resets dev-demo.db, seeds synthetic servers, metrics, traffic, uptime, and network probe history
• start in-memory demo agents that keep live metrics and network probe updates flowing locally
• add make dev-demo / make server-dev-demo commands and document SERVERBEE_DEV__DEMO_DATA

Test Plan

• cargo test -p serverbee-server dev_demo
• bun test scripts/make-menu.test.ts
• cargo clippy --all --benches --tests --examples --all-features