feat(dashboard): webhook rules status and test panel

[iceice400]

Summary

• GET /api/webhooks/status — rule list, last-fired timestamps, engine state (no full URLs)
• POST /api/webhooks/test/{rule_name} — dummy POST with event: test to verify connectivity from the Pi
• Configuration → Advanced → Webhooks panel with live status refresh and per-rule Test button

Why

Operators need visibility into webhook rules and a one-click way to verify LAN URLs without editing yaml or waiting for a real mesh event.

Depends on

PR #82 (webhook engine config). Merge #82 first, then rebase this branch onto main if needed.

Type

• Feature
• UI
• Bug fix
• Docs
• Refactor

Testing

• python -m unittest tests.test_webhooks_routes tests.test_webhook_engine -v (13 tests, pass)
• Dashboard tested on device

Test plan

1. Add webhook rules in local.yaml, open Configuration → Advanced → Webhooks
2. Confirm rules appear with host only (no full URL path with secrets)
3. Click Test — target receives JSON with event: test and data.test: true
4. Confirm Last fired updates immediately without service restart
5. Trigger a real event (e.g. low battery) — last-fired updates on next panel refresh

Relay / TX impact

None. Read-only status API + optional test POST; no pipeline changes beyond PR #82 engine tracking.

AI-assisted?

Implemented with AI assistance; unit tests run locally.