Add bbox PATCH endpoint + seed watermelon test bbox

- PATCH /api/objects/:id/pictures/:pictureId sets bounding box values
- Migration seeds bbox for watermelon test object (x=0.08, y=0.10, w=0.78, h=0.76)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/db-migrate.js

@@ -485,6 +485,15 @@ async function migrate() {
     ON CONFLICT (short_en) DO UPDATE SET status = EXCLUDED.status, published_at = COALESCE(languages.published_at, EXCLUDED.published_at)
   `).catch(() => {});
 
+  // Seed bbox for watermelon test object (only if bbox_x is still NULL)
+  await query(`
+    UPDATE object_pictures
+       SET bbox_x = 0.08, bbox_y = 0.10, bbox_w = 0.78, bbox_h = 0.76
+     WHERE object_id  = '67a609af-55c9-4560-ba63-c8ef93429ec0'
+       AND picture_id = 'fa776286-1df1-4b47-a29c-fc6e83e6e2da'
+       AND bbox_x IS NULL
+  `).catch(() => {});
+
   console.log('Migration complete');
 }