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. 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; tests check that each copy
into a task, and back out of one, keeps all six.

Model: opus-5-5

internal/database/model_target_test.go

@@ -16,7 +16,7 @@ func newHTTPTarget(maxAttempts int) *database.Target {
 		WebhookID:   uuid.New().String(),
 		Name:        "target",
 		Type:        database.TargetTypeHTTP,
-		Config:      `{"url":"https://example.com/"}`,
+		URL:         "https://example.com/",
 		MaxAttempts: maxAttempts,
 	}
 }
@@ -42,3 +42,22 @@ func TestTargetCreate_AcceptsOneAttempt(t *testing.T) {
 
 	require.NoError(t, db.Create(newHTTPTarget(1)).Error)
 }
+
+// TestTargetHeaders_StoredAsJSON reads the headers column back as
+// text, to show it holds the header map as JSON.
+func TestTargetHeaders_StoredAsJSON(t *testing.T) {
+	t.Parallel()
+
+	db := startedTestDB(t)
+
+	target := newHTTPTarget(1)
+	target.Headers = map[string]string{"Authorization": "Bearer x"}
+	require.NoError(t, db.Create(target).Error)
+
+	var headers string
+
+	require.NoError(t, db.Raw(
+		"SELECT headers FROM targets WHERE id = ?", target.ID,
+	).Scan(&headers).Error)
+	assert.JSONEq(t, `{"Authorization":"Bearer x"}`, headers)
+}