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/client_ssrf_test.go

@@ -46,12 +46,7 @@ func TestClientForRequest_TimeoutKeepsSSRFGuard(t *testing.T) {
 		t.Run(target, func(t *testing.T) {
 			t.Parallel()
 
-			cfg := &delivery.HTTPTargetConfig{
-				URL:     target,
-				Timeout: 5,
-			}
-
-			client := engine.ExportClientForRequest(cfg, nil)
+			client := engine.ExportClientForRequest(5, nil)
 
 			require.NotSame(t, engine.ExportClient(), client,
 				"a per-target timeout must yield a "+
@@ -101,11 +96,7 @@ func TestClientForRequest_NoTimeoutUnchanged(t *testing.T) {
 
 	engine := newSSRFTestEngine()
 
-	cfg := &delivery.HTTPTargetConfig{
-		URL: "https://example.com/hook",
-	}
-
-	client := engine.ExportClientForRequest(cfg, nil)
+	client := engine.ExportClientForRequest(0, nil)
 
 	assert.Same(t, engine.ExportClient(), client,
 		"without a per-target timeout the shared client "+
@@ -123,15 +114,8 @@ func TestClientForRequest_HeadersKeepSSRFGuard(t *testing.T) {
 
 	engine := newSSRFTestEngine()
 
-	cfg := &delivery.HTTPTargetConfig{
-		URL: "https://example.com/with-headers",
-		Headers: map[string]string{
-			"X-Api-Key": "configured",
-		},
-	}
-
 	client := engine.ExportClientForRequest(
-		cfg, []string{"X-Api-Key"},
+		0, []string{probeHeaderName},
 	)
 
 	require.NotNil(t, client.CheckRedirect,