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>

infra/schema.sql

@@ -125,3 +125,21 @@ CREATE INDEX IF NOT EXISTS figures_paper_idx ON figures (paper_id);
 
 -- F-16 Chunk Quality Gate: section classification
 ALTER TABLE chunks ADD COLUMN IF NOT EXISTS section TEXT;
+
+-- F-17 Paper identifier aliases: one paper → many OpenAlex IDs
+-- (arXiv preprint ID + journal published ID + any future version)
+-- Each OpenAlex ID maps to exactly one paper (UNIQUE on openalex_id).
+-- Populated automatically by ingest; add extra aliases manually.
+CREATE TABLE IF NOT EXISTS paper_identifiers (
+    paper_id    TEXT NOT NULL REFERENCES papers(id) ON DELETE CASCADE,
+    openalex_id TEXT NOT NULL,
+    PRIMARY KEY (paper_id, openalex_id)
+);
+
+CREATE UNIQUE INDEX IF NOT EXISTS paper_identifiers_oa_idx
+    ON paper_identifiers (openalex_id);
+
+-- Seed from existing papers.openalex_id
+INSERT INTO paper_identifiers (paper_id, openalex_id)
+SELECT id, openalex_id FROM papers WHERE openalex_id IS NOT NULL
+ON CONFLICT DO NOTHING;