Feature: Webhook notifications (Discord, Slack)

[chrisq-dev]

Send a webhook notification every time a credential is captured, configurable from the dashboard.\n\nSupported targets: Discord, Slack, custom URL\n\nAcceptance criteria:\n- [ ] Webhook URL configurable from dashboard (no reflash needed)\n- [ ] Payload includes: template, captured fields, timestamp\n- [ ] Retry on failure\n- [ ] Toggle on/off from dashboard

[chrisq-dev]

Status update: configurable ntfy/custom HTTP webhook support exists today. Keeping this issue open because retry behavior, explicit dashboard toggle, and provider-specific Discord/Slack payload polish are still pending from the original acceptance criteria.