feat(parsers+api): 270/271 eligibility request + response (SP3 P4)

Adds API-only 270/271 eligibility flow per spec section 3.4 (no UI, no
DB persistence). Operator pastes a 270 query, gets back raw 270 text,
submits manually, pastes the 271 back.

New models (270 + 271):
  - InformationSource, InformationReceiver, Subscriber, Patient
  - EligibilityBenefitInquiry (EQ) and CoverageBenefit (EB)
  - Inline SERVICE_TYPE_CODES dict (566 codes, snapshot 2026-06-20)
  - service_type_description() lookup helper

Parsers (single segment walker, defensive, mirrors 835 pattern):
  - parse_270: HL*20/21/22/23 loops + EQ, NM1, DMG, TRN
  - parse_271: HL*20/21/22/23 loops + EB, DTP, MSG, AAA

Serializer:
  - serialize_270: ISA/GS/ST/BHT/HL*20/21/22/patient/EQ/SE/GE/IEA
  - round-trips: serialize -> parse -> serialize == identical

API:
  - POST /api/eligibility/request (JSON body -> {raw_270_text, parsed})
  - POST /api/eligibility/parse-271 (file upload -> structured summary)
  - 400 on missing fields / garbage; never 500.

Tests: 365 -> 408 backend (+43 new), 10 frontend unchanged.
TypeScript clean.

backend/tests/test_parse_271.py

@@ -0,0 +1,98 @@
+"""Tests for the 271 (Eligibility Benefit Response) parser.
+
+SP3 Phase 4 (T21): single-pass segment walker that pulls the
+information source / receiver / subscriber / patient / EB coverage
+benefits out of a tokenized 271. The fixture
+(``tests/fixtures/minimal_271.txt``) has two EB segments with
+descriptions — one Medical Care, one Pharmacy — so we can assert
+both the count and the per-benefit resolution of service-type codes.
+"""
+from __future__ import annotations
+
+from pathlib import Path
+
+import pytest
+
+from cyclone.parsers.parse_271 import parse
+
+FIXTURE = Path(__file__).parent / "fixtures" / "minimal_271.txt"
+
+
+def test_parse_minimal_271_returns_two_benefits():
+    """The fixture's two EB segments become two CoverageBenefit rows."""
+    text = FIXTURE.read_text()
+    result = parse(text, input_file="minimal_271.txt")
+    assert len(result.coverage_benefits) == 2
+    # First EB: medical care (code "1"). Second: pharmacy (code "88").
+    assert result.coverage_benefits[0].service_type_code == "1"
+    assert result.coverage_benefits[1].service_type_code == "88"
+
+
+def test_parse_271_resolves_service_type_description():
+    """``service_type_description`` is auto-populated from the EB01 code."""
+    text = FIXTURE.read_text()
+    result = parse(text, input_file="minimal_271.txt")
+    assert result.coverage_benefits[0].service_type_description == "Medical Care"
+    assert result.coverage_benefits[1].service_type_description == "Pharmacy"
+
+
+def test_parse_271_in_plan_network_flag():
+    """The first EB has Y for in-plan, the second has N."""
+    text = FIXTURE.read_text()
+    result = parse(text, input_file="minimal_271.txt")
+    assert result.coverage_benefits[0].in_plan_network is True
+    assert result.coverage_benefits[1].in_plan_network is False
+
+
+def test_parse_271_envelope_built():
+    """Envelope has the ISA/GS/ST fields. ST03 = implementation guide."""
+    text = FIXTURE.read_text()
+    result = parse(text, input_file="minimal_271.txt")
+    env = result.envelope
+    assert env.sender_id.strip() == "PAYERID"
+    assert env.receiver_id.strip() == "PROVIDERID"
+    assert env.control_number == "000000002"
+    assert env.implementation_guide == "005010X279A1"
+    # GS04 supplies the transaction date.
+    assert env.transaction_date.year == 2024
+    assert env.transaction_date.month == 1
+    assert env.transaction_date.day == 1
+
+
+def test_parse_271_msg_folded_into_additional_info():
+    """MSG segments become ``BenefitAdditionalInfo`` rows on the EB."""
+    text = FIXTURE.read_text()
+    result = parse(text, input_file="minimal_271.txt")
+    cb0 = result.coverage_benefits[0]
+    assert len(cb0.additional_info) == 1
+    assert cb0.additional_info[0].description == "Patient has active coverage"
+    cb1 = result.coverage_benefits[1]
+    assert len(cb1.additional_info) == 1
+    assert cb1.additional_info[0].description == "Pharmacy copay $20"
+
+
+def test_parse_271_subscriber_extracted():
+    """Subscriber first/last name + member id come from NM1*IL."""
+    text = FIXTURE.read_text()
+    result = parse(text, input_file="minimal_271.txt")
+    assert result.subscriber.first_name == "JOHN"
+    assert result.subscriber.last_name == "DOE"
+    assert result.subscriber.member_id == "MEMBER123"
+
+
+def test_parse_271_invalid_edi_raises():
+    """Garbage input must raise :class:`CycloneParseError`."""
+    from cyclone.parsers.exceptions import CycloneParseError
+
+    with pytest.raises(CycloneParseError):
+        parse("not edi at all", input_file="garbage.txt")
+
+
+def test_parse_271_summary_counts_eb_segments():
+    """The summary reports the number of EB segments we parsed."""
+    text = FIXTURE.read_text()
+    result = parse(text, input_file="minimal_271.txt")
+    assert result.summary.total_claims == 2
+    assert result.summary.passed == 2
+    assert result.summary.failed == 0
+    assert result.summary.input_file == "minimal_271.txt"