feat(bookstack-api-5gi): implement Attachments CRUD

Add AttachmentsService with List, Get, Create, Update, Delete.
Support link attachments via JSON. Add Attachment type and request
types. Register service on Client. Add tests.

attachments.go

@@ -0,0 +1,56 @@
+package bookstack
+
+import (
+	"context"
+	"fmt"
+)
+
+// AttachmentsService handles operations on attachments.
+type AttachmentsService struct {
+	client *Client
+}
+
+// List returns a list of attachments with optional filtering.
+func (s *AttachmentsService) List(ctx context.Context, opts *ListOptions) ([]Attachment, error) {
+	var resp listResponse[Attachment]
+	err := s.client.do(ctx, "GET", "/api/attachments"+opts.queryString(), nil, &resp)
+	if err != nil {
+		return nil, err
+	}
+	return resp.Data, nil
+}
+
+// Get retrieves a single attachment by ID.
+func (s *AttachmentsService) Get(ctx context.Context, id int) (*Attachment, error) {
+	var a Attachment
+	err := s.client.do(ctx, "GET", fmt.Sprintf("/api/attachments/%d", id), nil, &a)
+	if err != nil {
+		return nil, err
+	}
+	return &a, nil
+}
+
+// Create creates a new link attachment.
+func (s *AttachmentsService) Create(ctx context.Context, req *AttachmentCreateRequest) (*Attachment, error) {
+	var a Attachment
+	err := s.client.do(ctx, "POST", "/api/attachments", req, &a)
+	if err != nil {
+		return nil, err
+	}
+	return &a, nil
+}
+
+// Update updates an existing attachment.
+func (s *AttachmentsService) Update(ctx context.Context, id int, req *AttachmentUpdateRequest) (*Attachment, error) {
+	var a Attachment
+	err := s.client.do(ctx, "PUT", fmt.Sprintf("/api/attachments/%d", id), req, &a)
+	if err != nil {
+		return nil, err
+	}
+	return &a, nil
+}
+
+// Delete deletes an attachment by ID.
+func (s *AttachmentsService) Delete(ctx context.Context, id int) error {
+	return s.client.do(ctx, "DELETE", fmt.Sprintf("/api/attachments/%d", id), nil, nil)
+}