Fail deliveries on archive errors; validate expiry at creation (#43)

Two review findings on the database archiving target:

- An archive error now records the attempt as failed with the
  error string and marks the delivery failed, instead of logging
  the error and reporting success. A target that could not do its
  one job must not claim it did.
- The archive expiry is now actually configurable: the add-target
  form gains an expiry field for database targets, and the value
  is validated at creation time via the new
  delivery.ValidateArchiveExpiry (empty, "never", or a positive
  Go duration), rejecting bad values with a 400 at the only place
  a human can fix them, mirroring how Slack target URLs are
  validated at creation.

Test updates: a forced archive failure asserts a failed delivery
with a recorded error and no archive file; config builder tests
cover empty/never/duration and rejection paths; the two engine
tests that exercise the database target now build engines with a
real webhook DB manager since archiving is no longer a no-op; the
reopen-debounce test uses a wider window so parallel test load
cannot make two rapid writes straddle it.

internal/delivery/engine_test.go

@@ -342,33 +342,29 @@ func TestDeliverDatabase_ImmediateSuccess(
 	t.Parallel()
 
 	db := testWebhookDB(t)
-	e := testEngine(t, 1)
 
-	event := seedEvent(t, db, `{"db":"target"}`)
-
-	dlv := seedDelivery(
-		t, db, event.ID, uuid.New().String(),
-		database.DeliveryStatusPending,
+	// The database target archives for real now, so the engine
+	// needs a webhook DB manager to locate the data directory.
+	e := delivery.NewTestEngineWithDB(
+		nil,
+		database.NewTestWebhookDBManager(t.TempDir()),
+		slog.New(slog.NewTextHandler(
+			os.Stderr,
+			&slog.HandlerOptions{Level: slog.LevelDebug},
+		)),
+		&http.Client{Timeout: 5 * time.Second},
+		1,
 	)
 
-	d := &database.Delivery{
-		EventID:  event.ID,
-		TargetID: dlv.TargetID,
-		Status:   database.DeliveryStatusPending,
-		Event:    event,
-		Target: database.Target{
-			Name: "test-db",
-			Type: database.TargetTypeDatabase,
-		},
-	}
-	d.ID = dlv.ID
+	event := seedEvent(t, db, `{"db":"target"}`)
+	d := seedDatabaseTargetDelivery(t, db, event, "")
 
 	e.ExportDeliverDatabase(db, d)
 
 	var updated database.Delivery
 
 	require.NoError(t, db.First(
-		&updated, "id = ?", dlv.ID,
+		&updated, "id = ?", d.ID,
 	).Error)
 
 	assert.Equal(t,
@@ -379,7 +375,7 @@ func TestDeliverDatabase_ImmediateSuccess(
 	var result database.DeliveryResult
 
 	require.NoError(t, db.Where(
-		"delivery_id = ?", dlv.ID,
+		"delivery_id = ?", d.ID,
 	).First(&result).Error)
 
 	assert.True(t, result.Success)
@@ -1158,7 +1154,19 @@ func TestProcessDelivery_RoutesToCorrectHandler(
 	t.Parallel()
 
 	db := testWebhookDB(t)
-	e := testEngine(t, 1)
+
+	// The database target archives for real now, so the engine
+	// needs a webhook DB manager to locate the data directory.
+	e := delivery.NewTestEngineWithDB(
+		nil,
+		database.NewTestWebhookDBManager(t.TempDir()),
+		slog.New(slog.NewTextHandler(
+			os.Stderr,
+			&slog.HandlerOptions{Level: slog.LevelDebug},
+		)),
+		&http.Client{Timeout: 5 * time.Second},
+		1,
+	)
 
 	tests := []struct {
 		name       string