Add per-delivery replay to the event log (closes #203)

A delivery that exhausted max_retries was failed forever. The event
body is durably stored, so the only way to get it delivered was to
download it and re-POST by hand.

The event log now offers a Replay action on any finished delivery.
Replay creates a NEW pending delivery for the same event and target
and hands it to the delivery engine through the same Notifier the
receiver uses, so it is retried, SSRF-guarded and circuit-broken
exactly as a first attempt. The original delivery's status,
timestamps and recorded attempts are never touched, and what is
re-sent is the stored event body, not the response the original
attempt received.

The target is read as it stands now, including soft-deleted rows so
that a deleted target refuses the replay with a message on the page
instead of erroring or delivering from stale configuration. A
deactivated target and a target id that names nothing refuse the same
way, as does a replay of a delivery the engine has not finished.

Two bounds on replay storms: the route carries a per-client POST rate
limit of 30 per minute, and the handler refuses a replay while an
earlier one for the same event and target is still pending or
retrying.

One new metric, webhooker_delivery_replays_total, on the existing
target_type label. A replay is a real delivery and moves the attempt,
outcome and duration series like any other; this counter is what
separates it from ordinary traffic without adding a dimension to
every existing series.

The delivery row is written with associations omitted and with
neither Event nor Target populated, so no target row reaches the
per-webhook event database.

internal/handlers/source_logs_test.go

@@ -60,10 +60,26 @@ func renderSourceLogsPage(
 ) string {
 	t.Helper()
 
+	return renderSourceLogsPageWithQuery(
+		t, h, sess, webhookID, "",
+	)
+}
+
+// renderSourceLogsPageWithQuery is renderSourceLogsPage over a
+// caller-supplied query string, for the page state a redirect back to
+// the log carries in one.
+func renderSourceLogsPageWithQuery(
+	t *testing.T,
+	h *handlers.Handlers,
+	sess *session.Session,
+	webhookID, query string,
+) string {
+	t.Helper()
+
 	req := httptest.NewRequestWithContext(
 		context.Background(),
 		http.MethodGet,
-		"/source/"+webhookID+"/logs",
+		"/source/"+webhookID+"/logs"+query,
 		nil,
 	)