Add a statistics pane to the webhook page (closes #368)

Each webhook's event database keeps one row of running totals: events,
deliveries and failures, and how many of each retention removed.
Storing an event, creating a delivery, a delivery becoming failed and
the retention sweep each update it in the transaction that writes or
deletes the rows it counts. Deliveries get a finished_at column, the
last column of the status index, so the last-10-minutes and
last-24-hours figures are index-range counts. The pane is its own
template, included at the top of the page.

The schema changes in place with nothing back-filled, so an existing
database must be recreated.

Model: opus-5-5

internal/handlers/handlers.go

@@ -91,18 +91,22 @@ type Handlers struct {
 
 // parsePageTemplate parses a page-specific template set from the
 // embedded FS. Each page template is combined with the shared
-// base, htmlheader, and navbar templates. The page file must be
-// listed first so that its root action ({{template "base" .}})
-// becomes the template set's entry point.
-func parsePageTemplate(pageFile string) *template.Template {
+// base, htmlheader, and navbar templates, and with any further files
+// the page includes. The page file must be listed first so that its
+// root action ({{template "base" .}}) becomes the template set's entry
+// point.
+func parsePageTemplate(
+	pageFile string, included ...string,
+) *template.Template {
+	files := append([]string{
+		pageFile,
+		"base.html",
+		"htmlheader.html",
+		"navbar.html",
+	}, included...)
+
 	return template.Must(
-		template.ParseFS(
-			templates.Templates,
-			pageFile,
-			"base.html",
-			"htmlheader.html",
-			"navbar.html",
-		),
+		template.ParseFS(templates.Templates, files...),
 	)
 }
 
@@ -131,7 +135,7 @@ func New(
 		"profile.html":       parsePageTemplate("profile.html"),
 		"sources_list.html":  parsePageTemplate("sources_list.html"),
 		"sources_new.html":   parsePageTemplate("sources_new.html"),
-		"source_detail.html": parsePageTemplate("source_detail.html"),
+		"source_detail.html": parsePageTemplate("source_detail.html", "webhook_stats.html"),
 		"source_edit.html":   parsePageTemplate("source_edit.html"),
 		"source_logs.html":   parsePageTemplate("source_logs.html"),
 		"target_edit.html":   parsePageTemplate("target_edit.html"),