refactor(store): code quality fixes per review

- iter_claims/iter_remittances now return list[dict] (snapshot under
  the lock, return outside) so slow consumers no longer hold the
  write lock during iteration
- iter_claims gains payer (case-insensitive substring) and
  date_from/date_to filters via the new _date_in_bounds helper
- iter_remittances wires date_from/date_to and payer
- BatchRecord split into BatchRecord837 / BatchRecord835 with
  __new__ dispatch so isinstance narrows result correctly,
  eliminating the four 'type: ignore[assignment]' band-aids
- BatchRecord now rejects naive parsed_at via model_validator
- to_ui_remittance's adjustmentAmount=0.0 is documented as a
  TODO for sub-project 2 reconciliation

151 passed, 1 skipped (no regressions; +6 tests for new filters)

backend/tests/test_store.py

@@ -485,3 +485,113 @@ def test_iter_remittances_fills_payer_name_from_batch():
     out = list(s.iter_remittances())
     assert len(out) == 1
     assert out[0]["payerName"] == "CO_TXIX"
+
+
+# ---------------------------------------------------------------------------
+# Review fixes: payer / date filters, tz-aware validator
+# ---------------------------------------------------------------------------
+
+
+def _make_claim_with_payer(claim_id: str, payer_name: str) -> ClaimOutput:
+    """ClaimOutput variant that lets us pick a payer name per row."""
+    c = _make_claim(claim_id)
+    c.payer = Payer(name=payer_name, id="P1")
+    return c
+
+
+def test_iter_claims_filters_by_payer_substring_case_insensitive():
+    """iter_claims(payer=...) is a case-insensitive substring match on payerName."""
+    s = InMemoryStore()
+    result = _make_result()
+    result.claims = [
+        _make_claim_with_payer("A", "Aetna Health"),
+        _make_claim_with_payer("B", "Blue Cross"),
+        _make_claim_with_payer("C", "aetna better health"),
+    ]
+    s.add(BatchRecord(
+        id="b1", kind="837p", input_filename="a.txt",
+        parsed_at=_PARSED_AT, result=result,
+    ))
+    out = s.iter_claims(payer="Aetna")
+    ids = sorted(c["id"] for c in out)
+    assert ids == ["A", "C"], f"expected Aetna matches, got {ids}"
+
+
+def test_iter_claims_filters_by_date_from_includes_in_range_only():
+    """date_from excludes items whose submissionDate is before the bound."""
+    s = InMemoryStore()
+    old = datetime(2026, 6, 10, 12, 0, 0, tzinfo=timezone.utc)
+    new = datetime(2026, 6, 25, 12, 0, 0, tzinfo=timezone.utc)
+    old_result = _make_result()
+    old_result.claims = [_make_claim("OLD")]
+    new_result = _make_result()
+    new_result.claims = [_make_claim("NEW")]
+    s.add(BatchRecord(
+        id="b-old", kind="837p", input_filename="a.txt",
+        parsed_at=old, result=old_result,
+    ))
+    s.add(BatchRecord(
+        id="b-new", kind="837p", input_filename="b.txt",
+        parsed_at=new, result=new_result,
+    ))
+    out = s.iter_claims(date_from="2026-06-20")
+    assert [c["id"] for c in out] == ["NEW"]
+
+
+def test_iter_claims_filters_by_date_to_includes_in_range_only():
+    """date_to excludes items whose submissionDate is after the bound."""
+    s = InMemoryStore()
+    old = datetime(2026, 6, 10, 12, 0, 0, tzinfo=timezone.utc)
+    new = datetime(2026, 6, 25, 12, 0, 0, tzinfo=timezone.utc)
+    old_result = _make_result()
+    old_result.claims = [_make_claim("OLD")]
+    new_result = _make_result()
+    new_result.claims = [_make_claim("NEW")]
+    s.add(BatchRecord(
+        id="b-old", kind="837p", input_filename="a.txt",
+        parsed_at=old, result=old_result,
+    ))
+    s.add(BatchRecord(
+        id="b-new", kind="837p", input_filename="b.txt",
+        parsed_at=new, result=new_result,
+    ))
+    out = s.iter_claims(date_to="2026-06-20")
+    assert [c["id"] for c in out] == ["OLD"]
+
+
+def test_iter_remittances_filters_by_date_from_and_date_to():
+    """Remit receivedDate respects both date_from and date_to bounds."""
+    s = InMemoryStore()
+    old = datetime(2026, 6, 10, 12, 0, 0, tzinfo=timezone.utc)
+    mid = datetime(2026, 6, 20, 12, 0, 0, tzinfo=timezone.utc)
+    new = datetime(2026, 6, 30, 12, 0, 0, tzinfo=timezone.utc)
+    for ts, tag in [(old, "OLD"), (mid, "MID"), (new, "NEW")]:
+        s.add(BatchRecord(
+            id=f"835-{tag}", kind="835", input_filename="a.txt",
+            parsed_at=ts, result=_make_835_result(claims=[_make_remit()]),
+        ))
+    out = s.iter_remittances(date_from="2026-06-15", date_to="2026-06-25")
+    assert [r["batchId"] for r in out] == ["835-MID"]
+
+
+def test_iter_claims_unbounded_date_filter_includes_all():
+    """With no date bounds, the filter is a no-op (all items returned)."""
+    s = InMemoryStore()
+    result = _make_result()
+    result.claims = [_make_claim("A"), _make_claim("B")]
+    s.add(BatchRecord(
+        id="b1", kind="837p", input_filename="a.txt",
+        parsed_at=_PARSED_AT, result=result,
+    ))
+    out = s.iter_claims()
+    assert len(out) == 2
+
+
+def test_batch_record_rejects_naive_parsed_at():
+    """tz-aware validator: a naive datetime should raise ValueError."""
+    import pytest
+    with pytest.raises(ValueError, match="tz-aware"):
+        BatchRecord(
+            id="x", kind="837p", input_filename="a.txt",
+            parsed_at=datetime(2025, 1, 1), result=_make_result(),
+        )