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/photosync/run_test.go

@@ -161,7 +161,7 @@ func syncEnv(t *testing.T) (config.Config, *store.DB, *stubS3, []string) {
 	}); err != nil {
 		t.Fatal(err)
 	}
-	if err := db.MarkPhotoReady(ctx, photoID, thumbKey, previewKey, 100, 80, now); err != nil {
+	if err := db.MarkPhotoReady(ctx, photoID, thumbKey, previewKey, 7, 100, 80, now); err != nil {
 		t.Fatal(err)
 	}
 	return cfg, db, stub, []string{origKey, thumbKey, previewKey}