Show an event's entrypoint and request headers (closes #389)

An expanded event in the event log and the event's own page now show
the entrypoint the event arrived at (its description, "Entrypoint"
when it has none, or "deleted entrypoint"; never its URL) and the
full request headers, one per line, sorted by name. Both pages draw
them through one shared template, event_request.html. The event
columns now load entrypoint_id and headers, and the webhook's
entrypoints are read once per page.

Model: opus-5-5

internal/handlers/handlers.go

@@ -166,10 +166,12 @@ func New(
 		),
 		"source_edit.html": parsePageTemplate("source_edit.html"),
 		"source_logs.html": parsePageTemplate(
-			"source_logs.html", "event_body.html", "delivery_attempts.html",
+			"source_logs.html", "event_request.html", "event_body.html",
+			"delivery_attempts.html",
 		),
 		"event_detail.html": parsePageTemplate(
-			"event_detail.html", "event_body.html", "delivery_attempts.html",
+			"event_detail.html", "event_request.html", "event_body.html",
+			"delivery_attempts.html",
 		),
 		"target_edit.html": parsePageTemplate("target_edit.html"),
 		"error.html":       parsePageTemplate("error.html"),