feat(sp28): store.claim_acks facade + batch_envelope_index + to_ui_claim_ack serializer

The persistence layer for the SP28 join table lands in
cyclone/store/claim_acks.py:

- add_claim_ack — insert + publish 'claim_ack_written' on the bus
  (mirrors the publish-from-store pattern used by the ACKs paths)
- list_acks_for_claim / list_claims_for_ack — read helpers for the
  two list endpoints
- find_ack_orphans — Inbox ack-orphans lane source: every ack row
  whose ack_id has no ClaimAck row tied to it
- remove_claim_ack — unlink + publish 'claim_ack_dropped'
- batch_envelope_index — D10 in-memory map of
  {envelope.control_number: batch.id}, called once per ingest (cost
  is ~16 lookups today; trivially cheap)

Plus the to_ui_claim_ack serializer in store/ui.py mirroring
to_ui_ack shape — single source of truth for the wire format so the
live-tail event payload matches the list endpoint byte-for-byte.
Includes a single SELECT against claims for the claim_state field
(TA1 batch-level rows return 'n/a').

The CycloneStore facade in store/__init__.py re-exports all six
methods (add_claim_ack, list_acks_for_claim, list_claims_for_ack,
find_ack_orphans, remove_claim_ack, batch_envelope_index) plus
to_ui_claim_ack from the ui module.

Migration-version assertions in test_acks.py and test_db_migrate.py
bumped 17 → 18 to match the new migration head.

Steps 3.1/3.2/3.3/3.4 of the SP28 implementation plan.

backend/tests/test_db_migrate.py

@@ -123,14 +123,15 @@ def test_migration_latest_idempotent_on_fresh_db(tmp_path: Path) -> None:
     index (drift-check perf).
     SP27 Task 17 bumped it to 17 with the patient_control_number
     backfill UPDATE (the migration runner now applies DML too).
+    SP28 bumped it to 18 with the claim_acks join table.
     """
     engine = _fresh_engine(tmp_path)
     db_migrate.run(engine)
     v_after_first = _user_version(engine)
-    assert v_after_first == 17, f"expected head=17, got {v_after_first}"
+    assert v_after_first == 18, f"expected head=18, got {v_after_first}"
 
     db_migrate.run(engine)
-    assert _user_version(engine) == 17, "second run should not bump version"
+    assert _user_version(engine) == 18, "second run should not bump version"
 
 
 def test_drop_claims_unique_constraint_migration(tmp_path: Path, monkeypatch: pytest.MonkeyPatch) -> None:
@@ -156,7 +157,7 @@ def test_drop_claims_unique_constraint_migration(tmp_path: Path, monkeypatch: py
     engine = _fresh_engine(tmp_path)
 
     db_migrate.run(engine)
-    assert _user_version(engine) == 17, f"expected head=17, got {_user_version(engine)}"
+    assert _user_version(engine) == 18, f"expected head=18, got {_user_version(engine)}"
 
     # Two claims in one batch with the same patient_control_number
     # must be insertable. If 0015's table recreation re-introduced a