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.

main.go

@@ -45,13 +45,19 @@ func main() {
 			os.Exit(1)
 		}
 		runTest(lm, os.Args[2], os.Args[3:])
+	case "template":
+		if len(os.Args) < 4 {
+			fmt.Fprintln(os.Stderr, "usage: campaigns template NAME PATH")
+			os.Exit(1)
+		}
+		runTemplate(lm, os.Args[2], os.Args[3])
 	default:
 		usage()
 	}
 }
 
 func usage() {
-	fmt.Fprintln(os.Stderr, "usage: campaigns <sync PATH|send SLUG|test SLUG EMAIL...>")
+	fmt.Fprintln(os.Stderr, "usage: campaigns <sync PATH|send SLUG|test SLUG EMAIL...|template NAME PATH>")
 	os.Exit(1)
 }
 
@@ -101,6 +107,14 @@ func runTest(lm *listmonk.Client, slug string, emails []string) {
 	log.Printf("test: sent preview of %q to %v", slug, emails)
 }
 
+func runTemplate(lm *listmonk.Client, name, path string) {
+	tpl, err := campaign.SyncTemplate(lm, name, path)
+	if err != nil {
+		log.Fatalf("template: %v", err)
+	}
+	log.Printf("template: %q (id=%d) is now the default campaign template", tpl.Name, tpl.ID)
+}
+
 func mustEnv(key string) string {
 	v := os.Getenv(key)
 	if v == "" {