Track altbody in change-detection so existing campaigns actually re-sync

Campaign/parseCampaign never fetched altbody, so campaignUnchanged()
compared everything except it -- a campaign synced before v0.1.3
(altbody always null) whose content otherwise hasn't changed would be
judged unchanged forever and never get updated with the new altbody,
even though every subsequent sync now intends to send one.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

internal/campaign/sync_test.go

@@ -31,6 +31,7 @@ type fakeCampaign struct {
 	Status     string
 	Subject    string
 	Body       string
+	AltBody    string
 	FromEmail  string
 	TemplateID int
 	Type       string
@@ -138,7 +139,7 @@ func (c fakeCampaign) toJSON() map[string]any {
 	}
 	return map[string]any{
 		"id": c.ID, "name": c.Name, "status": c.Status, "subject": c.Subject,
-		"body": c.Body, "from_email": c.FromEmail, "template_id": c.TemplateID,
+		"body": c.Body, "altbody": c.AltBody, "from_email": c.FromEmail, "template_id": c.TemplateID,
 		"type": c.Type, "tags": c.Tags, "lists": lists, "media": media,
 	}
 }
@@ -155,7 +156,7 @@ func (f *fakeListmonk) createCampaign(w http.ResponseWriter, r *http.Request) {
 	body := decodeBody(r)
 	c := fakeCampaign{
 		ID: f.id(), Name: str(body["name"]), Status: "draft",
-		Subject: str(body["subject"]), Body: str(body["body"]),
+		Subject: str(body["subject"]), Body: str(body["body"]), AltBody: str(body["altbody"]),
 		FromEmail: str(body["from_email"]), TemplateID: toInt(body["template_id"]),
 		Type: str(body["type"]), Tags: toStrings(body["tags"]),
 		ListIDs: toInts(body["lists"]), MediaIDs: toInts(body["media"]),
@@ -171,6 +172,7 @@ func (f *fakeListmonk) updateCampaign(w http.ResponseWriter, r *http.Request) {
 		if f.campaigns[i].ID == id {
 			f.campaigns[i].Subject = str(body["subject"])
 			f.campaigns[i].Body = str(body["body"])
+			f.campaigns[i].AltBody = str(body["altbody"])
 			f.campaigns[i].FromEmail = str(body["from_email"])
 			f.campaigns[i].TemplateID = toInt(body["template_id"])
 			f.campaigns[i].Type = str(body["type"])