feat(graph): paper_identifiers table — multi-ID alias support

Fixes preprint/journal duplicate problem: arXiv papers were ingested
under their preprint OpenAlex ID, but corpus citations reference the
published-version OpenAlex ID → those citations were invisible in the
graph (appeared as dangling).

Changes:
- infra/schema.sql: CREATE TABLE paper_identifiers (paper_id, openalex_id)
  with UNIQUE index; seeded from papers.openalex_id on migration
- codex/graph.py: build_citation_graph LEFT JOINs paper_identifiers as
  a second resolution path: COALESCE(p.id, pi.paper_id, c.cited_id)
- codex/ingest.py: every ingest inserts openalex_id into paper_identifiers
  (ON CONFLICT DO NOTHING) — aliases can be added manually alongside

Live DB: 35 rows seeded + 4 published-version aliases added:
  math/0503219 → W2163787581 (DCG 2007)
  math/0306167 → W2136126748 (Commun Analysis Geom 2004)
  math/0203250 → W1567166970 (Trans AMS 2003)
  1005.2698    → W1511400044 (Geom & Topol 2015)

Effect: dangling 574→570; Bobenko+Springborn 2007 now IN-KB rank 9.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

codex/ingest.py

@@ -147,6 +147,19 @@ def ingest_paper(
             },
         )
 
+        # Register openalex_id in paper_identifiers (alias table).
+        # Every ingest records the primary openalex_id so the graph JOIN
+        # resolves it even if papers.openalex_id is later updated.
+        if paper.openalex_id:
+            conn.execute(
+                """
+                INSERT INTO paper_identifiers (paper_id, openalex_id)
+                VALUES (%s, %s)
+                ON CONFLICT DO NOTHING
+                """,
+                (paper.id, paper.openalex_id),
+            )
+
         # ---------------------------------------------------------------
         # 4. Parse + chunk + embed source file (if provided)
         #    DELETE existing chunks for this paper first, then bulk INSERT