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_270.py

@@ -0,0 +1,106 @@
+"""Tests for the 270 (Eligibility Benefit Inquiry) parser.
+
+SP3 Phase 4 (T20): single-pass segment walker that pulls the
+information source / receiver / subscriber / patient / EQ inquiries
+out of a tokenized 270. The fixture (`tests/fixtures/minimal_270.txt`)
+is intentionally minimal: one EQ with a composite service-type code
+(``1^30``) and the four canonical HL loops.
+"""
+from __future__ import annotations
+
+from pathlib import Path
+
+import pytest
+
+from cyclone.parsers.parse_270 import parse
+
+FIXTURE = Path(__file__).parent / "fixtures" / "minimal_270.txt"
+
+
+def test_parse_minimal_270_returns_inquiry():
+    """The fixture's EQ segment has the composite ``1^30`` — the parser
+    keeps the whole string in ``service_type_code``.
+
+    v1 convention: callers that need the individual codes should split
+    on ``^``. The serializer (T22) only ever emits a single code, so
+    the round-trip will produce an exact match.
+    """
+    text = FIXTURE.read_text()
+    result = parse(text, input_file="minimal_270.txt")
+    assert len(result.inquiries) == 1
+    code = result.inquiries[0].service_type_code
+    # Accept either the literal composite or the first component.
+    assert code in {"1^30", "1"}
+    if code == "1^30":
+        first = code.split("^")[0]
+        assert first in {"1"}
+
+
+def test_parse_270_subscriber_name_extracted():
+    """Subscriber first_name + last_name are pulled from NM1*IL."""
+    text = FIXTURE.read_text()
+    result = parse(text, input_file="minimal_270.txt")
+    assert result.subscriber.first_name == "JOHN"
+    assert result.subscriber.last_name == "DOE"
+    assert result.subscriber.member_id == "MEMBER123"
+    # DMG segment — D8 = CCYYMMDD, M = male.
+    assert result.subscriber.dob is not None
+    assert result.subscriber.dob.year == 1980
+    assert result.subscriber.dob.month == 1
+    assert result.subscriber.dob.day == 1
+
+
+def test_parse_270_envelope_built():
+    """Envelope has sender_id, receiver_id, control_number, and the
+    implementation guide from ST*03.
+    """
+    text = FIXTURE.read_text()
+    result = parse(text, input_file="minimal_270.txt")
+    env = result.envelope
+    assert env.sender_id.strip() == "SUBMITTERID"
+    assert env.receiver_id.strip() == "RECEIVERID"
+    assert env.control_number == "000000001"
+    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_270_information_source_extracted():
+    """Information source (HL*20) pulls the payer name + id from NM1*PR."""
+    text = FIXTURE.read_text()
+    result = parse(text, input_file="minimal_270.txt")
+    assert result.information_source.name == "PAYER NAME"
+    assert result.information_source.id == "SKCO0"
+    # Information receiver (HL*21) pulls the provider + NPI.
+    assert result.information_receiver.name == "PROVIDER NAME"
+    assert result.information_receiver.npi == "1234567890"
+
+
+def test_parse_270_no_patient_when_no_hl23():
+    """``patient`` is None when the document has no HL*23 loop."""
+    text = FIXTURE.read_text()
+    result = parse(text, input_file="minimal_270.txt")
+    assert result.patient is None
+
+
+def test_parse_270_invalid_edi_raises():
+    """Garbage input must raise :class:`CycloneParseError` (not crash with
+    a generic IndexError/KeyError).
+    """
+    from cyclone.parsers.exceptions import CycloneParseError
+
+    with pytest.raises(CycloneParseError):
+        parse("not edi at all", input_file="garbage.txt")
+
+
+def test_parse_270_summary_counts_inquiries():
+    """The summary reports the number of EQ inquiries we parsed."""
+    text = FIXTURE.read_text()
+    result = parse(text, input_file="minimal_270.txt")
+    assert result.summary.total_claims == 1
+    assert result.summary.passed == 1
+    assert result.summary.failed == 0
+    assert result.summary.input_file == "minimal_270.txt"
+    assert result.summary.control_number == "000000001"