Store a target's settings as columns (closes #543)

A target's settings move from the per-type JSON string in
targets.config to columns: url, headers, timeout_seconds,
forward_query, archive_expiry and archive_rotation. A column a type
does not use stays empty. headers holds the header map as JSON text
through GORM's JSON serializer, and archive_expiry holds a
time.Duration, 0 meaning never, so nothing that reads a target parses
it. The forms check each setting once, with the same refusals and
messages, and the edit form is filled from the columns, so the
per-type config structs, their parsers and the "could not be read"
branches are gone. A delivery task carries the six settings in place
of the JSON string.

Model: opus-5-5

internal/delivery/redirect_test.go

@@ -85,19 +85,14 @@ func deliverWithProbeHeaders(
 		database.DeliveryStatusPending,
 	)
 
-	cfg, err := json.Marshal(delivery.HTTPTargetConfig{
-		URL: targetURL,
-		Headers: map[string]string{
-			probeHeaderName: probeHeaderValue,
-		},
-	})
-	require.NoError(t, err)
-
 	body := event.Body
 	task := iTask(
 		d, event, s.WebhookID, targetID,
-		"redirect-target", string(cfg), 1, 1, &body,
+		"redirect-target", targetURL, 1, 1, &body,
 	)
+	task.TargetHeaders = map[string]string{
+		probeHeaderName: probeHeaderValue,
+	}
 
 	s.Engine.ExportProcessNewTask(context.TODO(), &task)
 
@@ -309,8 +304,7 @@ func TestRedirectPolicy_StopsAtHopCap(t *testing.T) {
 	)
 
 	client := engine.ExportClientForRequest(
-		&delivery.HTTPTargetConfig{URL: srv.URL},
-		[]string{probeHeaderName},
+		0, []string{probeHeaderName},
 	)
 	require.NotNil(t, client.CheckRedirect)
 
@@ -350,7 +344,7 @@ func TestApplyRequestHeaders_ReportsOriginScopedNames(t *testing.T) {
 	inbound, err := json.Marshal(map[string][]string{
 		inboundHeaderName: {inboundHeaderValue},
 		"Content-Type":    {testContentType},
-		"User-Agent":      {"curl/8.7.1"},
+		headerUserAgent:   {"curl/8.7.1"},
 		"Host":            {"inbound.example.com"},
 	})
 	require.NoError(t, err)
@@ -369,11 +363,9 @@ func TestApplyRequestHeaders_ReportsOriginScopedNames(t *testing.T) {
 			Headers:     string(inbound),
 			ContentType: testContentType,
 		},
-		&delivery.HTTPTargetConfig{
-			Headers: map[string]string{
-				probeHeaderName: probeHeaderValue,
-				"Content-Type":  testContentType,
-			},
+		map[string]string{
+			probeHeaderName: probeHeaderValue,
+			"Content-Type":  testContentType,
 		},
 		"webhooker/dev",
 	)