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/handlers/target_toggle_test.go

@@ -60,7 +60,5 @@ func TestHandleTargetToggle_DoesNotUndoAnEdit(t *testing.T) {
 	assert.False(t, stored.Active)
 	assert.Equal(t, "edited-name", stored.Name)
 	assert.Equal(t, 5, stored.MaxAttempts)
-	assert.Equal(
-		t, editReplacedURL, storedHTTPConfig(t, env, tgt.ID).URL,
-	)
+	assert.Equal(t, editReplacedURL, stored.URL)
 }