feat(web): add automatic actions for open pull requests

A new pr_open alert carries the head commit SHA of each open, non-draft, same-repository pull request. An action with auto enabled sends its webhook once for each pull request and head commit. An authors list limits which pull requests trigger it. The alert never changes repository health.

internal/web/api.go

@@ -95,6 +95,7 @@ type apiAlert struct {
 	Subject      string                     `json:"subject"`
 	Detail       string                     `json:"detail"`
 	URL          string                     `json:"url"`
+	Ref          string                     `json:"ref,omitempty"`
 }
 
 func seconds(d time.Duration) int64 { return int64(d / time.Second) }
@@ -133,7 +134,7 @@ func buildState(v collector.View, manualRefresh bool) apiState {
 	for _, a := range v.Alerts {
 		st.Alerts = append(st.Alerts, apiAlert{
 			Severity: a.Severity.String(), Repo: a.Repo, Kind: a.Kind, Subject: a.Subject,
-			Detail: a.Detail, URL: a.URL, Since: a.Since,
+			Detail: a.Detail, URL: a.URL, Ref: a.Ref, Since: a.Since,
 		})
 	}
 	return st