fix(backend): clear a re-uploaded photo by id, not id+slab

slab_photos.id is a global primary key and photo ids are derived from the slab
id, so scoping the replace to the slab let a retried or concurrent upload of
the same draft hit UNIQUE(id) and 500. Clear the id unconditionally.

backend/photos.py

@@ -59,8 +59,10 @@ async def upload_photo(
     proc_bytes = await processed.read() if processed is not None else None
     if not orig_bytes:
         return JSONResponse(status_code=400, content=_error("validation", "original photo is empty"))
-    # Replace any existing photo with the same id on this slab (re-upload).
-    conn.execute("DELETE FROM slab_photos WHERE id=? AND slab_id=?", (photo_id, slab_id))
+    # slab_photos.id is a global primary key and photo ids are derived from the
+    # slab id, so a re-upload must clear the id regardless of slab: a retried or
+    # concurrent upload of the same draft would otherwise hit UNIQUE(id).
+    conn.execute("DELETE FROM slab_photos WHERE id=?", (photo_id,))
     seq_val = seq if seq is not None else _next_seq(conn, slab_id)
     conn.execute(
         "INSERT INTO slab_photos(id, slab_id, seq, is_calibration, role, original_blob, processed_blob, created_at) "