fix(embeddings): key_share alone is FOR NO KEY UPDATE, not FOR KEY SHARE (#3262)

SQLAlchemy spells Postgres's four row locks as a read/key_share pair, so
`with_for_update(key_share=True)` renders FOR NO KEY UPDATE — an
exclusive lock that two refreshes of the same record would fight over,
and that an ordinary concurrent edit would block. The claim needs
`read=True` as well to be the FOR KEY SHARE the docstring describes.

Caught by the unit test that compiles the statement, which is the whole
reason it asserts on the rendered lock mode rather than on behaviour
that looks identical either way.

src/scribe/services/embeddings.py

@@ -374,7 +374,10 @@ async def _claim_parent_row(session, id_column, row_id: int, label: str) -> bool
         held = (await session.execute(
             select(id_column)
             .where(id_column == row_id)
-            .with_for_update(key_share=True, nowait=True)
+            # BOTH flags: SQLAlchemy spells the four Postgres row locks as a
+            # read/key_share pair, and key_share alone is FOR NO KEY UPDATE —
+            # which would make two refreshes of one record fight each other.
+            .with_for_update(read=True, key_share=True, nowait=True)
         )).scalar_one_or_none()
     except Exception:
         # LockNotAvailable: this record is being deleted right now. Not an