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/user_agent_test.go

@@ -2,7 +2,6 @@ package delivery_test
 
 import (
 	"context"
-	"encoding/json"
 	"net/http"
 	"net/http/httptest"
 	"net/netip"
@@ -59,9 +58,9 @@ func TestUserAgent_IsTheBuildVersion(t *testing.T) {
 
 	statusCode, _, _, err := e.ExportDoHTTPRequest(
 		context.Background(),
-		&delivery.HTTPTargetConfig{
+		&database.Target{
 			URL:     ts.URL,
-			Headers: map[string]string{"User-Agent": "configured/1"},
+			Headers: map[string]string{headerUserAgent: "configured/1"},
 		},
 		&database.Event{Headers: `{"User-Agent":["curl/8"]}`},
 	)
@@ -73,18 +72,13 @@ func TestUserAgent_IsTheBuildVersion(t *testing.T) {
 	db := testWebhookDB(t)
 	targetID := uuid.New().String()
 
-	slackCfg, err := json.Marshal(
-		delivery.SlackTargetConfig{WebhookURL: ts.URL},
-	)
-	require.NoError(t, err)
-
 	event := seedEvent(t, db, `{"action":"test"}`)
 	dlv := seedDelivery(
 		t, db, event.ID, targetID, database.DeliveryStatusPending,
 	)
 
 	e.ExportDeliverSlack(context.Background(), db, buildSlackDelivery(
-		dlv, event, targetID, "test-slack", string(slackCfg),
+		dlv, event, targetID, "test-slack", ts.URL,
 	))
 	require.Len(t, userAgents, 1, "the slack target sent no request")
 	assert.Equal(t, want, <-userAgents, "slack target")