feat(sp37): populate batches.transaction_set_control_number on 837P ingest

The parser now captures the source 837's ST02 (transaction set control
number) on the Envelope model as 'transaction_set_control_number'.
add_record reads it off the envelope and stores it on the Batch row,
mirroring how 'envelope.control_number' (ISA13) was already handled.

Unlocks the SP37 join-key update so 999 set_control_number (AK201)
values resolve back to the right batch via Pass 1.

backend/tests/test_batch_txn_set_cn.py

@@ -0,0 +1,103 @@
+"""SP37 Task 2: add_record populates Batch.transaction_set_control_number.
+
+The column should mirror the source 837 envelope's
+``transaction_set_control_number`` (ST02) so the join-key update in
+Task 3 can resolve 999 ``set_control_number`` (AK201) values back to
+the right batch. The 835 path leaves the column NULL because the
+field is an 837P-specific join key (835 remittances don't need to
+resolve back to a source 837 — they're the response side of the loop).
+"""
+from __future__ import annotations
+
+from datetime import date, datetime, timezone
+from pathlib import Path
+
+from cyclone import db as db_mod
+from cyclone.db import Batch
+from cyclone.parsers.parse_837 import parse as parse_837_text
+from cyclone.parsers.payer import PayerConfig
+from cyclone.store import store as cycl_store
+from cyclone.store.records import BatchRecord837, BatchRecord835
+
+FIXTURE_837 = Path(__file__).parent / "fixtures" / "minimal_837p.txt"
+FIXTURE_835 = Path(__file__).parent / "fixtures" / "minimal_835.txt"
+
+
+def test_add_record_populates_transaction_set_control_number_for_837():
+    """parse_837 → add_record writes the parsed ST02 onto the Batch row.
+
+    The fixture's ST02 (``991102977``) must round-trip into the new
+    ``batches.transaction_set_control_number`` column. This is the
+    join key Task 3's 999-ingest Pass 1 update relies on.
+    """
+    text = FIXTURE_837.read_text()
+    parsed = parse_837_text(text, PayerConfig.co_medicaid())
+
+    record = BatchRecord837(
+        id="b-txn-cn-1",
+        input_filename="minimal_837p.txt",
+        parsed_at=datetime.now(timezone.utc),
+        result=parsed,
+    )
+    cycl_store.add(record)
+
+    with db_mod.SessionLocal()() as s:
+        row = s.get(Batch, "b-txn-cn-1")
+        assert row is not None
+        assert row.transaction_set_control_number == parsed.envelope.transaction_set_control_number
+        # Sanity: the fixture's ST02 must be the parsed-and-stored value.
+        assert row.transaction_set_control_number == "991102977"
+
+
+def test_envelope_model_exposes_transaction_set_control_number():
+    """The Envelope Pydantic model carries the new field with a None default.
+
+    Guards against a regression where someone removes the field from
+    the model — the rest of the chain (parser + ORM + write path)
+    silently degrades to None if the model loses the attribute.
+    """
+    from cyclone.parsers.models import Envelope
+
+    env = Envelope(
+        sender_id="S",
+        receiver_id="R",
+        control_number="000000001",
+        transaction_date=date(2026, 1, 1),
+    )
+    assert env.transaction_set_control_number is None
+
+    env2 = env.model_copy(update={"transaction_set_control_number": "0001"})
+    assert env2.transaction_set_control_number == "0001"
+
+
+def test_add_record_leaves_835_column_null():
+    """835 batches don't carry an ST02 join key; column stays NULL.
+
+    The shared ``Envelope`` class is used by both 837P and 835 parsers,
+    but only ``parse_837`` populates ``transaction_set_control_number``.
+    For 835 records the field is None and ``add_record`` writes NULL
+    to the column — verified end-to-end via a real 835 ingest.
+    """
+    text = FIXTURE_835.read_text()
+    from cyclone.parsers.parse_835 import parse as parse_835_text
+    parsed835 = parse_835_text(text, payer_config=None)  # type: ignore[arg-type]
+
+    # Sanity: the 835 envelope also has the field (shared model class),
+    # but the parser doesn't populate it — so it must be None.
+    assert parsed835.envelope.transaction_set_control_number is None
+
+    record = BatchRecord835(
+        id="b-txn-cn-835-1",
+        input_filename="minimal_835.txt",
+        parsed_at=datetime.now(timezone.utc),
+        result=parsed835,
+    )
+    cycl_store.add(record)
+
+    with db_mod.SessionLocal()() as s:
+        row = s.get(Batch, "b-txn-cn-835-1")
+        assert row is not None
+        assert row.transaction_set_control_number is None
+        # And the kind/kind round-trip is correct (sanity check that
+        # we did exercise the 835 path, not the 837 path).
+        assert row.kind == "835"