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/database/model_secrets_test.go

@@ -29,8 +29,10 @@ func marshalModel(t *testing.T, v any) string {
 // these tags the first handler that marshals a model serialises the
 // secret with it. Each field below is a live credential:
 //
-//   - Target.Config holds an incoming-webhook URL whose path segments
-//     are the bearer token.
+//   - Target.URL is an incoming-webhook URL whose path segments are
+//     the bearer token.
+//   - Target.Headers holds header values, routinely authorization
+//     tokens.
 //   - APIKey.Key is a bearer token outright.
 //   - Setting.Value holds the session encryption key.
 //   - User.Password holds the Argon2 hash, and was already tagged.
@@ -44,11 +46,20 @@ func TestModelsDoNotMarshalTheirSecrets(t *testing.T) {
 		model any
 	}{
 		{
-			name: "target config",
+			name: "target url",
 			model: database.Target{
 				Name:        keptField,
 				Type:        database.TargetTypeSlack,
-				Config:      `{"webhookUrl":"https://h/s/` + marker + `"}`,
+				URL:         "https://h/s/" + marker,
+				MaxAttempts: 1,
+			},
+		},
+		{
+			name: "target headers",
+			model: database.Target{
+				Name:        keptField,
+				Type:        database.TargetTypeHTTP,
+				Headers:     map[string]string{"Authorization": marker},
 				MaxAttempts: 1,
 			},
 		},
@@ -87,10 +98,10 @@ func TestModelsDoNotMarshalTheirSecrets(t *testing.T) {
 	}
 }
 
-// TestWebhookMarshalsNoTargetConfig covers the nested case: a webhook
+// TestWebhookMarshalsNoTargetURL covers the nested case: a webhook
 // marshalled with its targets preloaded must not carry the credential
 // through the association either.
-func TestWebhookMarshalsNoTargetConfig(t *testing.T) {
+func TestWebhookMarshalsNoTargetURL(t *testing.T) {
 	t.Parallel()
 
 	const marker = "QQNESTEDMARKERQQ"
@@ -99,7 +110,7 @@ func TestWebhookMarshalsNoTargetConfig(t *testing.T) {
 		Name: keptField,
 		Targets: []database.Target{{
 			Name:        "slack",
-			Config:      `{"webhookUrl":"https://h/s/` + marker + `"}`,
+			URL:         "https://h/s/" + marker,
 			MaxAttempts: 1,
 		}},
 	})