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/model/model.go

@@ -90,6 +90,8 @@ type PullRequest struct {
 	Checks    CheckSummary
 	Number    int
 	Draft     bool
+	// Fork is true when the head repository differs from the base or was deleted.
+	Fork bool
 }
 
 type CheckSummary struct {
@@ -133,12 +135,16 @@ func (r Run) Failed() bool {
 }
 
 type Alert struct {
-	Since    time.Time
-	Repo     string
-	Kind     string
-	Subject  string
-	Detail   string
-	URL      string
+	Since   time.Time
+	Repo    string
+	Kind    string
+	Subject string
+	Detail  string
+	URL     string
+	// Ref is the head commit of a pr_open alert; empty for other kinds.
+	Ref string
+	// Author is the pull request author of a pr_open alert; it is not part of the JSON API.
+	Author   string
 	Severity Severity
 }