feat(sp8): GET /api/claims/{id}/serialize-837 endpoint

Returns the persisted ClaimOutput as a regenerated X12 837P file via
the new outbound serializer (Approach A — full rebuild from canonical
fields). 404 on missing claim, 422 if the stored raw_json cannot be
validated as a ClaimOutput. text/x12 content-type, attachment
disposition with the claim id as the filename.

3 tests:
- endpoint returns text/x12 attachment starting with ISA*
- 404 for missing claim id
- regenerated text round-trips back through parse()

backend/tests/test_api_serialize_837.py

@@ -0,0 +1,54 @@
+"""Tests for the GET /api/claims/{id}/serialize-837 endpoint (SP8)."""
+import io
+from pathlib import Path
+
+from fastapi.testclient import TestClient
+
+from cyclone.api import app
+
+
+def _seed_claim(client: TestClient) -> str:
+    fixture = Path("tests/fixtures/co_medicaid_837p.txt").read_text()
+    r = client.post(
+        "/api/parse-837",
+        files={"file": ("claim.txt", io.BytesIO(fixture.encode()), "text/plain")},
+        headers={"Accept": "application/json"},
+    )
+    assert r.status_code == 200, r.text
+    claim_id = r.json()["claims"][0]["claim_id"]
+    return claim_id
+
+
+def test_endpoint_returns_x12_attachment():
+    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
+    assert r.headers["content-type"].startswith("text/x12")
+    cd = r.headers["content-disposition"]
+    assert "attachment" in cd
+    assert f"claim-{claim_id}.x12" in cd
+    assert r.text.startswith("ISA*")
+
+
+def test_endpoint_404_for_missing_claim():
+    with TestClient(app) as client:
+        r = client.get("/api/claims/CLM-DOES-NOT-EXIST/serialize-837")
+    assert r.status_code == 404
+    body = r.json()
+    assert body["error"] == "Not found"
+    assert "CLM-DOES-NOT-EXIST" in body["detail"]
+
+
+def test_endpoint_regenerated_text_round_trips_through_parser():
+    """The endpoint's body should be a parseable 837 — round-trip test."""
+    from cyclone.parsers.parse_837 import parse
+    from cyclone.parsers.payer import PayerConfig
+
+    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
+    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