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

Each webhook's event database keeps running totals: one row for its
events, and one row per target for that target's deliveries, delivered
and failed, each with what retention removed. Every write to them
shares the transaction of the rows it counts. Deliveries get a
finished_at column; it and target_id end the status index, so each
target's deliveries finished in a window come from one index-range
query grouped by target. Retention deletes 1000 expired events per
transaction, pausing 200 ms between them so other writers get in. The
pane is its own template, its figures in tables.

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"),