feat(webhooks): configurable outbound HTTP rules for mesh events

[iceice400]

Summary

• Adds configurable outbound HTTP webhooks driven by webhooks.rules in local.yaml
• Async POST via httpx on mesh events: battery low, node offline/online, keyword match, duty spike
• Per-rule cooldown in memory; failures logged to admin audit log (webhook.fire)
• Disabled by default — no behaviour change until webhooks.enabled: true

Why

Operators want Home Assistant / Node-RED / Slack-style integrations without polling the dashboard API. Webhooks fire off the decoded packet stream without touching relay or TX paths.

Type

• Feature
• Bug fix
• Docs
• Refactor
• UI
• Installer
• Region support
• Hardware change

Testing

• python -m unittest tests.test_webhook_engine -v (9 tests, pass)
• Tested on hardware with real webhook target

Test plan

1. Default config → service starts; no outbound HTTP
2. Enable one battery_low rule pointing at a test HTTP server; inject low-battery telemetry → POST received
3. Repeat within cooldown → single POST only
4. Kill webhook target → packet pipeline continues; audit log shows webhook.fire error
5. Invalid config (duplicate name, missing keyword, bad URL) → startup rejects with clear error

Relay / TX impact

None. Webhook engine subscribes to decoded packets only; no relay/decoder changes.

Config

webhooks:
  enabled: false
  rules:
    - name: low-battery-ha
      url: "http://192.168.1.10:8123/api/webhook/mesh_low_battery"
      event: battery_low
      cooldown_seconds: 3600

storm_quarantine validates at startup but does not fire until storm-guard (PR 12) lands.

AI-assisted?

Implemented with AI assistance; unit tests run locally.