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/graph.py

@@ -47,9 +47,11 @@ def build_citation_graph(conn: Any) -> nx.DiGraph:
     """
     rows = conn.execute(
         """
-        SELECT c.citing_id, COALESCE(p.id, c.cited_id) AS cited_id
+        SELECT c.citing_id, COALESCE(p.id, pi.paper_id, c.cited_id) AS cited_id
         FROM citations c
         LEFT JOIN papers p ON p.openalex_id = c.cited_id
+        LEFT JOIN paper_identifiers pi
+               ON pi.openalex_id = c.cited_id AND p.id IS NULL
         """
     ).fetchall()
     g: nx.DiGraph = nx.DiGraph()