fix(pmc): use per-PMID elink instead of batch to preserve ID mapping

Batch elink merges all source IDs into one LinkSet — no way to map
which PMC ID belongs to which PMID. Send one at a time instead.
Also removes defunct NCBI ID Converter API (returning 404).

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

ingestion/pmc.py

@@ -23,12 +23,15 @@ def _configure_entrez() -> None:
 
 
 def _sleep() -> None:
+    # NCBI rate limit: 10 req/s with API key, 3 req/s without
     time.sleep(0.1 if os.getenv("NCBI_API_KEY") else 0.4)
 
 
 def get_pmcids(pmids: list[str]) -> dict[str, str]:
     """
     Map PubMed IDs to PMC IDs for papers with Open Access full text.
+    Sends one PMID at a time to elink — batch elink merges all results
+    into one LinkSet with no per-ID mapping, making it unusable for this purpose.
     Returns {pmid: pmcid}.
     """
     _configure_entrez()
@@ -36,30 +39,28 @@ def get_pmcids(pmids: list[str]) -> dict[str, str]:
         return {}
 
     result: dict[str, str] = {}
-    for i in range(0, len(pmids), 200):
-        batch = pmids[i : i + 200]
+    for pmid in pmids:
         for attempt in range(3):
             try:
-                handle = Entrez.elink(dbfrom="pubmed", db="pmc", id=",".join(batch))
+                handle = Entrez.elink(dbfrom="pubmed", db="pmc", id=pmid)
                 link_sets = Entrez.read(handle)
                 handle.close()
                 break
             except Exception as exc:
                 if attempt == 2:
-                    _logger.warning(f"elink failed: {exc}")
+                    _logger.debug(f"elink failed for PMID {pmid}: {exc}")
                     link_sets = []
                     break
                 time.sleep(2 ** attempt)
 
-        for link_set in link_sets:
-            source_ids = link_set.get("IdList", [])
-            source_id = str(source_ids[0]) if source_ids else None
-            for db_link in link_set.get("LinkSetDb", []):
+        for ls in link_sets:
+            for db_link in ls.get("LinkSetDb", []):
                 if db_link.get("DbTo") == "pmc":
                     links = db_link.get("Link", [])
-                    if links and source_id:
-                        result[source_id] = str(links[0]["Id"])
+                    if links:
+                        result[pmid] = str(links[0]["Id"])
                     break
+
         _sleep()
 
     _logger.info("PMC ID lookup", extra={"data": {"pmids": len(pmids), "found": len(result)}})