Add mobile save sheet with same-origin photo downloads.

Phones cannot land downloads in the photo library, so the gallery opens a share sheet for a cacheable preview while streaming via a dedicated download endpoint and recording preview sizes.

Co-authored-by: Cursor <cursoragent@cursor.com>

photo-api/internal/httpapi/dto.go

@@ -42,6 +42,13 @@ type photoURLs struct {
 	Thumb    string `json:"thumb,omitempty"`
 	Preview  string `json:"preview,omitempty"`
 	Original string `json:"original"`
+	// Download / DownloadOriginal hit the download endpoint (files.go), which
+	// always streams the bytes same-origin instead of redirecting to S3, so a
+	// fetch() can read them into a Blob without CORS. Download serves the
+	// preview variant and doubles as the lightbox's <img> source, so the save
+	// fetch is answered from the HTTP cache rather than the network.
+	Download         string `json:"download,omitempty"`
+	DownloadOriginal string `json:"downloadOriginal"`
 }
 
 type photoJSON struct {
@@ -51,6 +58,9 @@ type photoJSON struct {
 	OriginalFilename string    `json:"originalFilename,omitempty"`
 	ContentType      string    `json:"contentType"`
 	SizeBytes        int64     `json:"sizeBytes"`
+	// PreviewSizeBytes lets the mobile save sheet label both of its rows with
+	// a real file size without fetching anything. Omitted while unknown.
+	PreviewSizeBytes int64     `json:"previewSizeBytes,omitempty"`
 	Width            int       `json:"width,omitempty"`
 	Height           int       `json:"height,omitempty"`
 	TakenAt          string    `json:"takenAt,omitempty"`
@@ -91,6 +101,22 @@ func fileURL(photoID, variant, token string) string {
 	return u
 }
 
+// downloadURL builds the download endpoint's URL. size is left off for the
+// preview, which the endpoint serves by default — a shorter, stabler string
+// for the URL the lightbox also renders.
+func downloadURL(photoID, size, token string) string {
+	u := "/api/photos/files/" + photoID + "/download"
+	sep := "?"
+	if size != "" && size != downloadSizePreview {
+		u += "?size=" + size
+		sep = "&"
+	}
+	if token != "" {
+		u += sep + "token=" + token
+	}
+	return u
+}
+
 func (s *Server) photoToJSON(p store.Photo, token string, admin bool) photoJSON {
 	out := photoJSON{
 		ID:               p.ID,
@@ -101,16 +127,21 @@ func (s *Server) photoToJSON(p store.Photo, token string, admin bool) photoJSON
 		SizeBytes:        p.SizeBytes,
 		Width:            p.Width,
 		Height:           p.Height,
+		PreviewSizeBytes: p.PreviewSizeBytes,
 		TakenAt:          isoTime(p.TakenAt),
 		Status:           p.Status,
 		CreatedAt:        isoTime(p.CreatedAt),
-		URLs:             photoURLs{Original: fileURL(p.ID, "original", token)},
+		URLs: photoURLs{
+			Original:         fileURL(p.ID, "original", token),
+			DownloadOriginal: downloadURL(p.ID, downloadSizeOriginal, token),
+		},
 	}
 	if p.ThumbKey != "" {
 		out.URLs.Thumb = fileURL(p.ID, "thumb", token)
 	}
 	if p.PreviewKey != "" {
 		out.URLs.Preview = fileURL(p.ID, "preview", token)
+		out.URLs.Download = downloadURL(p.ID, downloadSizePreview, token)
 	}
 	if admin {
 		out.LastError = p.LastError