Add template-as-code sync (campaigns template NAME PATH)

Answers "can the unsubscribe template be added via git+API too": commit
listmonk's own stock campaign template (already unsubscribe-capable) as
email-templates/campaign.html, add a small find-or-create-or-update
Template client (confirmed against knadh/listmonk's actual model/handlers),
a new `template` subcommand, and a manual-dispatch-only workflow to push it
in as the default campaign template. Unlike campaign sync there's no
draft/live status to protect, so this is always a safe overwrite.

internal/campaign/sync_test.go

@@ -44,6 +44,13 @@ type fakeMedia struct {
 	Filename string
 }
 
+type fakeTemplate struct {
+	ID        int
+	Name      string
+	Body      string
+	IsDefault bool
+}
+
 type testCall struct {
 	CampaignID int
 	Emails     []string
@@ -61,6 +68,7 @@ type fakeListmonk struct {
 	lists             []fakeList
 	campaigns         []fakeCampaign
 	media             []fakeMedia
+	templates         []fakeTemplate
 	testCalls         []testCall
 	segmentQueryCalls []segmentQueryCall
 	failSegmentQuery  bool
@@ -107,6 +115,12 @@ func (f *fakeListmonk) handle(w http.ResponseWriter, r *http.Request) {
 		f.uploadMedia(w, r)
 	case r.Method == http.MethodPut && r.URL.Path == "/api/subscribers/query/lists":
 		f.queryAddToList(w, r)
+	case r.Method == http.MethodGet && r.URL.Path == "/api/templates":
+		f.writeTemplates(w)
+	case r.Method == http.MethodPost && r.URL.Path == "/api/templates":
+		f.createTemplate(w, r)
+	case r.Method == http.MethodPut && strings.HasPrefix(r.URL.Path, "/api/templates/"):
+		f.updateTemplate(w, r)
 	default:
 		f.t.Errorf("fakeListmonk: unhandled request %s %s", r.Method, r.URL.Path)
 		w.WriteHeader(http.StatusNotFound)
@@ -244,6 +258,37 @@ func (f *fakeListmonk) queryAddToList(w http.ResponseWriter, r *http.Request) {
 	w.Write([]byte(`{}`))
 }
 
+func (f *fakeListmonk) writeTemplates(w http.ResponseWriter) {
+	results := make([]map[string]any, 0, len(f.templates))
+	for _, t := range f.templates {
+		results = append(results, map[string]any{"id": t.ID, "name": t.Name, "type": "campaign", "is_default": t.IsDefault})
+	}
+	json.NewEncoder(w).Encode(map[string]any{"data": results})
+}
+
+func (f *fakeListmonk) createTemplate(w http.ResponseWriter, r *http.Request) {
+	body := decodeBody(r)
+	isDefault, _ := body["is_default"].(bool)
+	tpl := fakeTemplate{ID: f.id(), Name: str(body["name"]), Body: str(body["body"]), IsDefault: isDefault}
+	f.templates = append(f.templates, tpl)
+	json.NewEncoder(w).Encode(map[string]any{"data": map[string]any{"id": tpl.ID}})
+}
+
+func (f *fakeListmonk) updateTemplate(w http.ResponseWriter, r *http.Request) {
+	id := pathID(r.URL.Path, "/api/templates/")
+	body := decodeBody(r)
+	for i := range f.templates {
+		if f.templates[i].ID == id {
+			f.templates[i].Name = str(body["name"])
+			f.templates[i].Body = str(body["body"])
+			f.templates[i].IsDefault, _ = body["is_default"].(bool)
+			w.Write([]byte(`{}`))
+			return
+		}
+	}
+	w.WriteHeader(http.StatusNotFound)
+}
+
 func decodeBody(r *http.Request) map[string]any {
 	var body map[string]any
 	b, _ := io.ReadAll(r.Body)