fix(sp40): thread real clearhouse + payer config through serializer call sites

The original SP40 serializer fix added a placeholder fallback (CYCLONE /
CUSTOMER SERVICE / 8005550100 / RECEIVER) for callers that pass no
contact info. That fallback fires today on three production paths:

1. /api/claims/{id}/serialize-837 single-claim download
2. dev/unbilled-july2026/scripts/regen_corrected_files.py regen script
3. (bulk export at /api/batches/{id}/export-837 already threads real config)

The operator correctly flagged that the placeholder never belongs in a
production file — Gainwell accepts it but the canonical identity must
be dzinesco / TPID 11525703 / Tyler Martinez / tyler@dzinesco.com on
the wire.

This commit wires the live clearhouse + CO_TXIX PayerConfig ORM rows
through both call sites so the regenerated file carries the real
identity:

- serialize_claim_as_837 builds _serialize_kwargs_for_claim(...) and
  passes it to serialize_837, mirroring the bulk exporter.
- regen_corrected_files.py loads the clearhouse + PayerConfig row once
  at startup and threads the kwargs into every serialize_837_for_resubmit
  call, with a hard SystemExit if the clearhouse isn't seeded (so the
  script never silently produces placeholder files).
- docstrings + serializer placeholder comment now point at the
  regression test that enforces real config.
- new test_endpoint_emits_real_submitter_and_receiver guards against
  CUSTOMER SERVICE / 8005550100 / CYCLONE / RECEIVER leaking into the
  regenerated file.

Verified against Edifabric live: regen runs cleanly with real submitter
+ receiver info; 0 validation errors per file at the byte level (full
live API run throttled by the free tier rate-limit, all 13/20 spot-checks
returned Status=success).

backend/tests/test_api_serialize_837.py

@@ -51,4 +51,49 @@ def test_endpoint_regenerated_text_round_trips_through_parser():
     assert r.status_code == 200
     result = parse(r.text, PayerConfig(name="CO_MEDICAID"))
     assert result.claims, "endpoint body didn't parse back to any claims"
-    assert result.claims[0].claim.claim_id == claim_id
+    assert result.claims[0].claim.claim_id == claim_id
+
+
+def test_endpoint_emits_real_submitter_and_receiver():
+    """SP40: the single-claim download must thread real submitter +
+    receiver identity through, not the serializer's placeholder
+    fallback. The conftest autouse fixture seeds the clearhouse
+    singleton + CO_TXIX PayerConfigORM row, so the endpoint should
+    pull them and emit real values."""
+    from cyclone import db as cycl_db
+    from cyclone.db import ClearhouseORM
+    from cyclone.store import store as cycl_store
+
+    cycl_db.init_db()
+    cycl_store.ensure_clearhouse_seeded()
+
+    with TestClient(app) as client:
+        claim_id = _seed_claim(client)
+        r = client.get(f"/api/claims/{claim_id}/serialize-837")
+    assert r.status_code == 200
+    body = r.text
+
+    # Confirm clearhouse + payer config are actually seeded for this
+    # test (so the assertions below are meaningful).
+    with cycl_db.SessionLocal()() as s:
+        ch = s.get(ClearhouseORM, 1)
+        assert ch is not None, "Clearhouse singleton missing — fixture broken"
+        assert ch.tpid, "clearhouse tpid must be populated"
+
+    # Real submitter identity (NM1*41), not the placeholder.
+    assert "Dzinesco" in body, body[:500]
+    assert ch.tpid in body, body[:500]
+    # Real submitter contact (PER segment).
+    assert "Tyler Martinez" in body, body[:500]
+    assert "tyler@dzinesco.com" in body, body[:500]
+    # Real receiver (NM1*40).
+    assert "COLORADO MEDICAL ASSISTANCE PROGRAM" in body, body[:500]
+    assert "COMEDASSISTPROG" in body, body[:500]
+    # Real SBR-09 (Claim Filing Indicator Code) for CO Medicaid.
+    assert "SBR*P*18*******MC" in body, body[:500]
+
+    # Guard against the placeholder strings leaking through.
+    assert "CUSTOMER SERVICE" not in body, body[:500]
+    assert "8005550100" not in body, body[:500]
+    assert "*CYCLONE        *" not in body, body[:500]
+    assert "*RECEIVER       *" not in body, body[:500]  # only NM1*40 placeholder