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/routes/objects.js

@@ -220,6 +220,20 @@ router.post('/:id/pictures/:pictureId', async (req, res, next) => {
   } catch (err) { next(err); }
 });
 
+// PATCH /api/objects/:id/pictures/:pictureId  — set bounding box (0–1 percentages)
+router.patch('/:id/pictures/:pictureId', async (req, res, next) => {
+  try {
+    const { bbox_x, bbox_y, bbox_w, bbox_h } = req.body;
+    await query(
+      `UPDATE object_pictures
+          SET bbox_x = $3, bbox_y = $4, bbox_w = $5, bbox_h = $6
+        WHERE object_id = $1 AND picture_id = $2`,
+      [req.params.id, req.params.pictureId, bbox_x ?? null, bbox_y ?? null, bbox_w ?? null, bbox_h ?? null]
+    );
+    res.status(204).end();
+  } catch (err) { next(err); }
+});
+
 // DELETE /api/objects/:id/pictures/:pictureId
 router.delete('/:id/pictures/:pictureId', async (req, res, next) => {
   try {