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>

codex/ingest.py

@@ -164,10 +164,26 @@ def ingest_paper(
                 )
                 break
             except psycopg.errors.UniqueViolation as exc:
-                if base_bibkey is None or "bibkey" not in str(exc).lower() or attempt == 26:
-                    raise
+                msg = str(exc).lower()
                 conn.rollback()
+                if "openalex" in msg:
+                    # Two different papers.id claim the same OpenAlex work — a real data
+                    # conflict, not something to auto-rename. Surface it clearly instead
+                    # of the raw psycopg error (companion gap to C-15's bibkey case).
+                    raise ValueError(
+                        f"openalex_id {paper.openalex_id!r} is already attached to a "
+                        f"different paper; ingesting '{paper.id}' would duplicate it — "
+                        "resolve the conflicting paper first."
+                    ) from exc
+                if base_bibkey is None or "bibkey" not in msg or attempt == 26:
+                    raise
                 paper.bibkey = f"{base_bibkey}{chr(ord('a') + attempt)}"
+                logger.warning(
+                    "bibkey %r already exists; ingesting '%s' as %r instead",
+                    base_bibkey,
+                    paper.id,
+                    paper.bibkey,
+                )
 
         # Register openalex_id in paper_identifiers (alias table).
         # Every ingest records the primary openalex_id so the graph JOIN