Fix spurious re-sync/preview for campaigns without an explicit template_id

campaignUnchanged compared input.TemplateID (0 when unset) literally
against existing.TemplateID, but listmonk always assigns a real, nonzero
template_id server-side even when create/update omits it. Every campaign
without an explicit template_id was therefore flagged "changed" on every
sync, forever, firing an unwanted extra preview email each time — caught
via reground-campaigns' welcome/campaign.md re-previewing on an unrelated
push. Treat TemplateID 0 as "don't compare" instead. Test fake now mirrors
listmonk's real default-assignment behavior so this class of bug is caught.

internal/campaign/sync.go

@@ -222,7 +222,13 @@ func campaignUnchanged(existing *listmonk.Campaign, input listmonk.CampaignInput
 		// stored altbody is still null even though its body hasn't changed.
 		existing.AltBody == input.Body &&
 		existing.FromEmail == input.FromEmail &&
-		existing.TemplateID == input.TemplateID &&
+		// input.TemplateID == 0 means "unspecified, use whatever's assigned"
+		// (see CampaignInput.TemplateID) -- listmonk always assigns some real,
+		// nonzero template_id server-side even when it's omitted from the
+		// create/update payload, so comparing 0 against that would flag every
+		// campaign.md without an explicit template_id as changed on every
+		// sync, forever.
+		(input.TemplateID == 0 || existing.TemplateID == input.TemplateID) &&
 		existing.Type == input.Type &&
 		equalStringSets(existing.Tags, input.Tags) &&
 		equalIntSets(existing.ListIDs, input.ListIDs) &&

internal/campaign/sync_test.go

@@ -152,12 +152,25 @@ func (f *fakeListmonk) writeCampaigns(w http.ResponseWriter) {
 	json.NewEncoder(w).Encode(map[string]any{"data": map[string]any{"results": results}})
 }
 
+// fakeDefaultTemplateID is the nonzero template_id real listmonk assigns
+// server-side to a campaign whose create/update request omitted template_id
+// entirely -- never 0, which is what makes the zero value ambiguous between
+// "unspecified" and "really is 0" in campaignUnchanged.
+const fakeDefaultTemplateID = 1
+
+func (f *fakeListmonk) resolveTemplateID(body map[string]any) int {
+	if v, ok := body["template_id"]; ok {
+		return toInt(v)
+	}
+	return fakeDefaultTemplateID
+}
+
 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"]), AltBody: str(body["altbody"]),
-		FromEmail: str(body["from_email"]), TemplateID: toInt(body["template_id"]),
+		FromEmail: str(body["from_email"]), TemplateID: f.resolveTemplateID(body),
 		Type: str(body["type"]), Tags: toStrings(body["tags"]),
 		ListIDs: toInts(body["lists"]), MediaIDs: toInts(body["media"]),
 	}
@@ -174,7 +187,7 @@ func (f *fakeListmonk) updateCampaign(w http.ResponseWriter, r *http.Request) {
 			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].TemplateID = f.resolveTemplateID(body)
 			f.campaigns[i].Type = str(body["type"])
 			f.campaigns[i].Tags = toStrings(body["tags"])
 			f.campaigns[i].ListIDs = toInts(body["lists"])
@@ -409,6 +422,39 @@ func TestSyncDir_SecondSyncWithUnchangedContentIsNoop(t *testing.T) {
 	}
 }
 
+func TestSyncDir_UnspecifiedTemplateIDDoesNotTriggerSpuriousResync(t *testing.T) {
+	f := newFakeListmonk(t)
+	f.lists = []fakeList{{ID: 3, Name: "Newsletter"}}
+	lm := f.client()
+
+	root := t.TempDir()
+	// baseFrontmatter never sets template_id, matching real campaign.md files
+	// like reground-campaigns' welcome/campaign.md.
+	writeCampaignDir(t, root, "launch", baseFrontmatter, "Hello world.")
+
+	if _, err := SyncDir(lm, root, nil); err != nil {
+		t.Fatalf("first SyncDir: %v", err)
+	}
+	if f.campaigns[0].TemplateID != fakeDefaultTemplateID {
+		t.Fatalf("expected the fake to assign its default template_id like real listmonk does, got %d", f.campaigns[0].TemplateID)
+	}
+
+	// Re-sync with the exact same, still-unspecified template_id: this must
+	// not be flagged as changed just because listmonk's stored, server-
+	// assigned template_id (nonzero) doesn't literally equal the frontmatter's
+	// unset value (0).
+	result, err := SyncDir(lm, root, nil)
+	if err != nil {
+		t.Fatalf("second SyncDir: %v", err)
+	}
+	if len(result.Synced) != 0 {
+		t.Errorf("expected no spurious re-sync from an unspecified template_id, got Synced=%v", result.Synced)
+	}
+	if len(result.Unchanged) != 1 || result.Unchanged[0] != "launch" {
+		t.Errorf("expected launch in Unchanged, got %v", result.Unchanged)
+	}
+}
+
 func TestSyncDir_UpdatesWhenContentChanges(t *testing.T) {
 	f := newFakeListmonk(t)
 	f.lists = []fakeList{{ID: 3, Name: "Newsletter"}}