Add thumbnail support to Canvas and storage backends

Introduces a 'thumbnail' field to the Canvas model and updates all storage backends (AWS S3, filesystem, memory, and SQLite) to handle storing and retrieving this field. Also updates the API handler to accept and save the thumbnail, and switches SQLite driver to modernc.org/sqlite for improved compatibility. Updates .gitignore to exclude .db files.

core/canvas.go

@@ -11,6 +11,7 @@ type (
 		ID        string    `json:"id"`
 		UserID    string    `json:"-"` // Not exposed in JSON responses, used internally.
 		Name      string    `json:"name"`
+		Thumbnail string    `json:"thumbnail,omitempty"`
 		Data      []byte    `json:"-"` // The full canvas data, not included in list views.
 		CreatedAt time.Time `json:"createdAt"`
 		UpdatedAt time.Time `json:"updatedAt"`