F17: replace merge_all with merge_one in upload_image and delete_image

Single-activity writes now trigger a fast merge_one instead of a full
user rebuild. post_activity was fixed earlier; this completes the fix
for upload_image and delete_image endpoints.

bincio/serve/server.py

@@ -592,8 +592,8 @@ async def post_activity(
 
     from bincio.edit.ops import apply_sidecar_edit
     body = await request.json()
+    # apply_sidecar_edit already calls merge_one internally — no full rebuild needed.
     apply_sidecar_edit(activity_id, body, dd)
-    _trigger_rebuild(user.handle)
     return JSONResponse({"ok": True})
 
 
@@ -633,7 +633,8 @@ async def upload_image(
     images_dir.mkdir(parents=True, exist_ok=True)
     safe_name = _unique_image_name(images_dir, Path(file.filename).name)
     (images_dir / safe_name).write_bytes(contents)
-    _trigger_rebuild(user.handle)
+    from bincio.render.merge import merge_one
+    merge_one(dd, activity_id)
     return JSONResponse({"ok": True, "filename": safe_name})
 
 
@@ -653,7 +654,8 @@ async def delete_image(
         target.unlink()
         if target.parent.exists() and not any(target.parent.iterdir()):
             shutil.rmtree(target.parent)
-    _trigger_rebuild(user.handle)
+    from bincio.render.merge import merge_one
+    merge_one(dd, activity_id)
     return JSONResponse({"ok": True})