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/template.go

@@ -0,0 +1,38 @@
+package campaign
+
+import (
+	"fmt"
+	"os"
+
+	"reground.org/eec-campaigns/internal/listmonk"
+)
+
+// SyncTemplate pushes a template file's content into listmonk under name,
+// creating it (as the default campaign template) if missing or overwriting
+// its body if it already exists. Unlike campaign sync, a template has no
+// send-lifecycle status to protect — there's no "draft vs. live" distinction
+// to guard, so this is always a safe create-or-update, no rejection path.
+func SyncTemplate(lm *listmonk.Client, name, path string) (*listmonk.Template, error) {
+	body, err := os.ReadFile(path)
+	if err != nil {
+		return nil, err
+	}
+
+	existing, err := lm.FindTemplateByName(name)
+	if err != nil {
+		return nil, err
+	}
+	if existing == nil {
+		created, err := lm.CreateTemplate(name, string(body), true)
+		if err != nil {
+			return nil, fmt.Errorf("creating template %q: %w", name, err)
+		}
+		return created, nil
+	}
+	if err := lm.UpdateTemplate(existing.ID, name, string(body), true); err != nil {
+		return nil, fmt.Errorf("updating template %q: %w", name, err)
+	}
+	existing.Body = string(body)
+	existing.IsDefault = true
+	return existing, nil
+}