fix(sp37): sync head-assertion + update batch_envelope_index docstrings

- test_acks.py:test_migration_latest_idempotent_on_fresh_db was still
  asserting user_version == 19; migration 0020 (Task 1) bumped it to 20.
  Without this fix, the full pytest suite fails the moment SP37 ships.
- claim_acks.py module docstring + store/__init__.py facade docstring
  still advertised the index as single-key {envelope.control_number:
  batch.id}. SP37 Task 3 made it dual-key (ISA13 + ST02); the docs now
  match the implementation.

backend/tests/test_acks.py

@@ -51,7 +51,7 @@ def test_migration_0002_creates_acks_table():
 
 def test_migration_latest_idempotent_on_fresh_db():
     """Re-running the migration on the same DB must be a no-op (PRAGMA
-    user_version already at the latest version — currently 19 after
+    user_version already at the latest version — currently 20 after
     0004-0006 line_reconciliation, 0005 ta1_acks, SP9's 0007
     providers/payers/clearhouse, SP10's 0008 payer_rejected,
     SP11's 0009 audit_log, SP14's 0010 payer_rejected_acknowledged,
@@ -61,15 +61,16 @@ def test_migration_latest_idempotent_on_fresh_db():
     claims.matched_remittance_id index, SP27-Task 17's 0017
     claim.patient_control_number backfill UPDATE, SP28's 0018
     claim_acks join table, SP32's 0019
-    rendering_provider_npi + service_provider_npi)."""
+    rendering_provider_npi + service_provider_npi, and SP37's 0020
+    transaction_set_control_number)."""
     with db.engine().begin() as c:
         v1 = c.exec_driver_sql("PRAGMA user_version").scalar() or 0
-    assert v1 == 19
+    assert v1 == 20
     # A second run should not raise and should not bump the version.
     db_migrate.run(db.engine())
     with db.engine().begin() as c:
         v2 = c.exec_driver_sql("PRAGMA user_version").scalar() or 0
-    assert v2 == 19
+    assert v2 == 20
 
 
 def test_add_ack_persists_row():