fix(ingest): make both upsert UNIQUE gaps visible (audit C-15 follow-up)

Per review: the two latent upsert conflicts should be observable even where not
auto-fixed.
- bibkey collision (auto-suffixed) now logs a warning so a paper landing as
  'BobenkoLutz2023a' instead of '…2023' isn't silent.
- openalex_id collision (two papers claiming the same OpenAlex work — a real data
  conflict, not auto-renamable) now raises a clear ValueError naming the
  openalex_id and the offending paper, instead of the raw psycopg UniqueViolation.

Regression test added for the openalex_id error path.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/ingest/test_ingest.py

@@ -560,6 +560,35 @@ def test_ingest_disambiguates_bibkey_on_unique_violation() -> None:
     mock_conn.rollback.assert_called_once()
 
 
+def test_ingest_openalex_collision_raises_clear_error() -> None:
+    """An openalex_id UNIQUE collision raises a clear, actionable error (audit C-15).
+
+    Unlike a bibkey clash (auto-suffixed), a shared openalex_id means two papers
+    claim the same OpenAlex work — a real data conflict that must surface, not the
+    raw psycopg error.
+    """
+    paper = _make_paper()
+    mock_conn = MagicMock()
+
+    def execute_side_effect(sql: str, params: Any = None) -> MagicMock:
+        if "INSERT INTO papers" in sql:
+            raise psycopg.errors.UniqueViolation(
+                'duplicate key value violates unique constraint "papers_openalex_id_key"'
+            )
+        return MagicMock()
+
+    mock_conn.execute.side_effect = execute_side_effect
+
+    with (
+        patch("codex.ingest.openalex.fetch_paper", return_value=paper),
+        patch("codex.ingest.openalex.fetch_citations", return_value=[]),
+        patch("codex.ingest.get_embedder", return_value=_fake_embedder()),
+        patch("codex.ingest.get_conn", side_effect=_make_conn_cm(mock_conn)),
+        pytest.raises(ValueError, match="openalex_id"),
+    ):
+        ingest_paper(paper.id)
+
+
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