feat(sp29): inbox rejected-row 999 ack chips + per-row Resubmit

- backend: _ack_summary_for_claims helper attaches claim_acks payload
  to inbox rejected-lane rows (3 tests)
- frontend: InboxRow renders newest 3 AK2 chips + '+N more' overflow
  under the rejection reason, with '999 not linked' marker when an
  ack couldn't be linked to a claim
- frontend: per-row Resubmit button downloads a single corrected 837
  via api.serializeClaim837 (no bulk modal, no zip — one click, one
  .x12 file)
- frontend: 2 new tests for the chip rendering and the per-row
  download flow

backend/tests/test_inbox_lanes.py

@@ -12,7 +12,7 @@ import pytest
 
 from cyclone import db
 from cyclone.db import (
-    Base, Batch, Claim, ClaimState, Remittance, init_db,
+    Base, Batch, Claim, Ack, ClaimAck, ClaimState, Remittance, init_db,
 )
 from cyclone.inbox_lanes import compute_lanes
 
@@ -142,3 +142,111 @@ def test_done_today_includes_recent_terminal_states():
     ids = {r["id"] for r in lanes.done_today}
     assert "C1" in ids
     assert "C2" not in ids
+
+
+# ---------------------------------------------------------------------------
+# SP29: rejected-lane rows must carry the per-claim 999 ack-evidence summary
+# so the Inbox can render AK2 chips inline + a per-row Resubmit button
+# without an extra round-trip.
+# ---------------------------------------------------------------------------
+
+def _add_999_ack(*, ack_id: int, parsed_at=None) -> None:
+    """Persist a minimal Ack row (the 999 envelope)."""
+    with db.SessionLocal()() as s:
+        if s.get(Ack, ack_id) is not None:
+            return
+        s.add(Ack(
+            id=ack_id, source_batch_id="B-1",
+            accepted_count=0, rejected_count=0, received_count=1,
+            ack_code="A",
+            parsed_at=parsed_at or datetime.now(timezone.utc),
+            raw_json={"set_responses": []},
+        ))
+        s.commit()
+
+
+def _link_claim_ack(
+    *, ack_id: int, claim_id: str, ak2_index: int,
+    set_accept_reject_code: str,
+    linked_at=None, set_control_number: str = "991102989",
+) -> None:
+    """Persist a single ClaimAck row linking an Ack to a Claim."""
+    with db.SessionLocal()() as s:
+        s.add(ClaimAck(
+            claim_id=claim_id, ack_id=ack_id, ack_kind="999",
+            ak2_index=ak2_index,
+            set_control_number=set_control_number,
+            set_accept_reject_code=set_accept_reject_code,
+            linked_at=linked_at or datetime.now(timezone.utc),
+            linked_by="auto",
+        ))
+        s.commit()
+
+
+def test_inbox_lanes_attaches_claim_acks_summary_for_rejected():
+    """A rejected-lane row gets a `claim_acks` field with total /
+    rejected counts + 5 most recent AK2 set_responses. Newest first.
+    """
+    _add_claim(claim_id="REJ-1", state=ClaimState.REJECTED)
+    # 3 linked 999 acks: 2 accepted, 1 rejected
+    _add_999_ack(ack_id=1)
+    _add_999_ack(ack_id=2)
+    _add_999_ack(ack_id=3)
+    t0 = datetime.now(timezone.utc)
+    _link_claim_ack(ack_id=1, claim_id="REJ-1", ak2_index=0,
+                    set_accept_reject_code="A",
+                    linked_at=t0 - timedelta(minutes=10))
+    _link_claim_ack(ack_id=2, claim_id="REJ-1", ak2_index=1,
+                    set_accept_reject_code="R",
+                    linked_at=t0 - timedelta(minutes=5))
+    _link_claim_ack(ack_id=3, claim_id="REJ-1", ak2_index=2,
+                    set_accept_reject_code="A",
+                    linked_at=t0 - timedelta(minutes=1))
+
+    with db.SessionLocal()() as s:
+        lanes = compute_lanes(s, dismissed_pairs=set())
+
+    rows = [r for r in lanes.rejected if r["id"] == "REJ-1"]
+    assert len(rows) == 1
+    row = rows[0]
+    assert "claim_acks" in row
+    summary = row["claim_acks"]
+    assert summary is not None
+    assert summary["total"] == 3
+    assert summary["rejected"] == 1
+    assert len(summary["items"]) == 3
+    # Newest first (descending linked_at)
+    assert summary["items"][0]["set_accept_reject_code"] == "A"   # ack 3 (most recent)
+    assert summary["items"][1]["set_accept_reject_code"] == "R"   # ack 2 (middle)
+    assert summary["items"][2]["set_accept_reject_code"] == "A"   # ack 1 (oldest)
+    # The R code is the one rejected entry
+    codes = [it["set_accept_reject_code"] for it in summary["items"]]
+    assert codes.count("R") == 1
+    # ak2_index is preserved
+    assert summary["items"][0]["ak2_index"] == 2
+    assert summary["items"][1]["ak2_index"] == 1
+
+
+def test_inbox_lanes_claim_acks_is_null_when_no_links():
+    """A rejected claim with zero linked 999 acks renders
+    `claim_acks: null` (the UI shows '999 not linked')."""
+    _add_claim(claim_id="REJ-2", state=ClaimState.REJECTED)
+    with db.SessionLocal()() as s:
+        lanes = compute_lanes(s, dismissed_pairs=set())
+    rows = [r for r in lanes.rejected if r["id"] == "REJ-2"]
+    assert len(rows) == 1
+    assert rows[0]["claim_acks"] is None
+
+
+def test_inbox_lanes_claim_acks_does_not_attach_to_other_lanes():
+    """The ack summary is only on `rejected`, not `payer_rejected` /
+    `unmatched` / `done_today` — those lanes have their own evidence
+    shape (payer_rejected_*, no linked 999s by construction)."""
+    _add_claim(claim_id="SUB-1", state=ClaimState.SUBMITTED)
+    _add_claim(claim_id="DEN-1", state=ClaimState.DENIED)
+    with db.SessionLocal()() as s:
+        lanes = compute_lanes(s, dismissed_pairs=set())
+    for r in lanes.unmatched:
+        assert "claim_acks" not in r
+    for r in lanes.done_today:
+        assert "claim_acks" not in r