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/listmonk/listmonk.go

@@ -499,3 +499,103 @@ func (c *Client) TestCampaign(id int, emails []string) error {
 	}
 	return nil
 }
+
+// ---- Templates ----
+
+// Template is the subset of listmonk's template fields eec-campaigns cares
+// about. Field names confirmed against knadh/listmonk's actual model
+// (models.Template: Name/Type/Body/IsDefault, json tags "name"/"type"/
+// "body"/"is_default") and handler routes (GET/POST /api/templates,
+// PUT /api/templates/:id).
+type Template struct {
+	ID        int
+	Name      string
+	Type      string
+	Body      string
+	IsDefault bool
+}
+
+func (c *Client) templatesAll() ([]Template, error) {
+	respBody, status, err := c.do(http.MethodGet, "/api/templates", nil)
+	if err != nil {
+		return nil, fmt.Errorf("listing templates: %w", err)
+	}
+	if status != http.StatusOK {
+		return nil, fmt.Errorf("listing templates failed (%d): %s", status, string(respBody))
+	}
+	var parsed struct {
+		Data []struct {
+			ID        int    `json:"id"`
+			Name      string `json:"name"`
+			Type      string `json:"type"`
+			Body      string `json:"body"`
+			IsDefault bool   `json:"is_default"`
+		} `json:"data"`
+	}
+	if err := json.Unmarshal(respBody, &parsed); err != nil {
+		return nil, fmt.Errorf("parsing template list response: %w", err)
+	}
+	templates := make([]Template, 0, len(parsed.Data))
+	for _, r := range parsed.Data {
+		templates = append(templates, Template{ID: r.ID, Name: r.Name, Type: r.Type, Body: r.Body, IsDefault: r.IsDefault})
+	}
+	return templates, nil
+}
+
+// FindTemplateByName returns (nil, nil) on a plain miss, matching
+// FindCampaignByName's convention.
+func (c *Client) FindTemplateByName(name string) (*Template, error) {
+	templates, err := c.templatesAll()
+	if err != nil {
+		return nil, err
+	}
+	var matches []Template
+	for _, t := range templates {
+		if t.Name == name {
+			matches = append(matches, t)
+		}
+	}
+	switch len(matches) {
+	case 0:
+		return nil, nil
+	case 1:
+		return &matches[0], nil
+	default:
+		return nil, fmt.Errorf("%d templates named %q — ambiguous, refusing to guess", len(matches), name)
+	}
+}
+
+// CreateTemplate creates a "campaign"-type template. isDefault: true makes
+// every new campaign use it without per-campaign template_id wiring.
+func (c *Client) CreateTemplate(name, body string, isDefault bool) (*Template, error) {
+	payload := map[string]any{"name": name, "type": "campaign", "body": body, "is_default": isDefault}
+	respBody, status, err := c.do(http.MethodPost, "/api/templates", payload)
+	if err != nil {
+		return nil, fmt.Errorf("creating template %q: %w", name, err)
+	}
+	if status != http.StatusOK {
+		return nil, fmt.Errorf("creating template %q failed (%d): %s", name, status, string(respBody))
+	}
+	var parsed struct {
+		Data struct {
+			ID int `json:"id"`
+		} `json:"data"`
+	}
+	if err := json.Unmarshal(respBody, &parsed); err != nil {
+		return nil, fmt.Errorf("parsing template creation response: %w", err)
+	}
+	return &Template{ID: parsed.Data.ID, Name: name, Type: "campaign", Body: body, IsDefault: isDefault}, nil
+}
+
+// UpdateTemplate overwrites an existing template's content.
+func (c *Client) UpdateTemplate(id int, name, body string, isDefault bool) error {
+	payload := map[string]any{"name": name, "type": "campaign", "body": body, "is_default": isDefault}
+	respBody, status, err := c.do(http.MethodPut, fmt.Sprintf("/api/templates/%d", id), payload)
+	if err != nil {
+		return fmt.Errorf("updating template %d: %w", id, err)
+	}
+	if status != http.StatusOK {
+		return fmt.Errorf("updating template %d failed (%d): %s", id, status, string(respBody))
+	}
+	return nil
+}