[Issue]: Add webhook and digest-based alert delivery options

[MrAdder]

Issue Type

Feature request

Summary

Expand alert delivery options with a generic webhook channel and digest-style alerting for noisy event streams.

Problem / Context

Mail, Slack, and Discord cover common use cases, but some teams need custom integrations or lower-noise delivery models.

Proposed Solution

Add a generic webhook alert channel and an optional digest or summary alert mode for repeated events.

Acceptance Criteria

• Alerts can be delivered through a generic webhook channel.
• Users can configure digest or summary alerting for repeated activity.
• Existing alert channels continue to work unchanged.

Implementation Notes

Review the current alert dispatcher and channel abstraction. Keep configuration backward-compatible.

Testing Notes

Add tests for webhook dispatch and digest behavior, including repeated-event scenarios.

Additional Context

This should extend integrations without forcing teams into existing channel choices.