fix(api): include adjustments array in /api/remittances list response (SP3 P2 follow-up)

The P2 subagent shipped a UI chevron+expansion in Remittances.tsx that
read adjustments from the list payload, but iter_remittances() didn't
include the CasAdjustment rows. Result: the expansion was always empty
when a remittance was opened from the list (the detail endpoint worked).

Fix: bulk-fetch all CasAdjustment rows for the remittance_ids in one
query, then attach them as the 'adjustments' array. N+1-free.

backend/tests/test_api_gets.py

@@ -149,6 +149,43 @@ def test_remittances_filters_by_batch_id(seeded_store):
     assert resp.json()["items"] == []
 
 
+# --------------------------------------------------------------------------- #
+# /api/remittances list payload includes adjustments array (SP3 P2 follow-up)
+# --------------------------------------------------------------------------- #
+
+
+def test_list_remittances_includes_adjustments_array(client: TestClient) -> None:
+    """SP3 P2 follow-up: the list endpoint must include `adjustments` so the
+    UI's expandable row has data when a remittance is opened from the list
+    (not just from the detail endpoint)."""
+    _seed_835_remittance(
+        pcns=["PCN-LIST-1"],
+        cas_rows={"PCN-LIST-1": [("CO", "45", "50.00", ""), ("PR", "1", "10.00", "")]},
+    )
+    resp = client.get("/api/remittances", headers=JSON)
+    assert resp.status_code == 200
+    body = resp.json()
+    assert len(body["items"]) == 1
+    item = body["items"][0]
+    assert "adjustments" in item, item
+    assert len(item["adjustments"]) == 2
+    # Verify labels come through (resolved by cas_codes.reason_label).
+    labels = {a["reason"]: a["label"] for a in item["adjustments"]}
+    assert "45" in labels and "1" in labels
+    assert labels["45"]  # non-empty
+    assert labels["1"]
+
+
+def test_list_remittances_empty_adjustments_is_empty_array(client: TestClient) -> None:
+    """A remittance with no CAS rows must still expose `adjustments: []`."""
+    _seed_835_remittance(pcns=["PCN-LIST-EMPTY"], cas_rows=None)
+    resp = client.get("/api/remittances", headers=JSON)
+    assert resp.status_code == 200
+    body = resp.json()
+    assert len(body["items"]) == 1
+    assert body["items"][0]["adjustments"] == []
+
+
 # --------------------------------------------------------------------------- #
 # /api/providers
 # --------------------------------------------------------------------------- #