Fix second-round YNAB review findings

- Failure backoff is re-checked under the sync lock, so parallel tool
  calls share one failed YNAB request instead of retrying in turn.
- Budget id: YNAB_BUDGET_ID aliases ("last-used", "default") count as
  unset; state is always keyed by the real budget id, a mismatched
  response errors instead of silently skipping every sync, and more
  than one stored budget is an error rather than an arbitrary pick.
- Change log renames only id fields that resolve to names (account,
  payee, category, group); subtransaction transaction_id stays as is.
- ynab_recent_changes: correct singular for "categories", multi-word
  types, and the "month" type.
- The SQL wrap test now parses the wrapped query.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/unit/test_ynab_mirror.py

@@ -1,8 +1,10 @@
 """Tests for the YNAB Postgres mirror: delta application, change log, SQL guard."""
 
+import asyncio
 from decimal import Decimal
 
 import pytest
+import sqlglot
 from tortoise import Tortoise
 from tortoise.transactions import in_transaction
 
@@ -248,7 +250,12 @@ def test_executed_sql_drops_comments_and_semicolon():
     )
     assert error is None
     assert "--" not in sql and not sql.endswith(";")
-    assert f"SELECT * FROM ({sql}) AS q"  # wraps cleanly
+    # The LIMIT wrapper must still parse: a leftover line comment would
+    # swallow its closing parenthesis
+    wrapped = sqlglot.parse_one(
+        f"SELECT * FROM ({sql}) AS q LIMIT 100", read="postgres"
+    )
+    assert wrapped.args["limit"] is not None
 
 
 async def test_full_sync_marks_missing_rows_deleted(db):
@@ -328,8 +335,14 @@ async def test_recent_changes_accepts_display_names(monkeypatch):
 
     monkeypatch.setattr(tools, "ensure_fresh", fresh)
     monkeypatch.setattr(tools, "_query", fake_query)
-    await tools.ynab_recent_changes.ainvoke({"entity_type": "Assignments"})
-    assert seen[-1][1] == "month_category"
+    for given, stored in [
+        ("Assignments", "month_category"),
+        ("categories", "category"),
+        ("category groups", "category_group"),
+        ("month", "month"),
+    ]:
+        await tools.ynab_recent_changes.ainvoke({"entity_type": given})
+        assert seen[-1][1] == stored
     result = await tools.ynab_recent_changes.ainvoke({"entity_type": "bogus"})
     assert result.startswith("Unknown entity_type")
 
@@ -345,3 +358,77 @@ async def test_budget_summary_rejects_bad_month(monkeypatch):
         assert await tools.ynab_budget_summary.ainvoke({"month": month}) == (
             "Month must be YYYY-MM."
         )
+
+
+async def test_subtransaction_parent_id_is_not_renamed(db):
+    split = {
+        "id": "sub-1",
+        "transaction_id": "txn-1",
+        "amount": -10_000,
+        "memo": None,
+        "payee_id": None,
+        "category_id": "cat-1",
+        "transfer_account_id": None,
+        "deleted": False,
+    }
+    budget = _budget([_txn()])
+    budget["subtransactions"] = [split]
+    await _apply(budget, 1, log_changes=False)
+    budget["subtransactions"] = [{**split, "amount": -12_000}]
+    await _apply(budget, 2)
+    change = await YnabChange.get(entity_type="subtransaction")
+    assert change.changes == {"amount": [-10.0, -12.0]}
+
+
+class TestBudgetPinning:
+    @pytest.fixture(autouse=True)
+    async def _db(self, db):
+        yield
+
+    async def _state(self, budget_id):
+        from blueprints.ynab.models import YnabSyncState
+
+        await YnabSyncState.create(budget_id=budget_id, server_knowledge=1)
+
+    @pytest.mark.parametrize("configured", ["", "last-used", "default"])
+    async def test_alias_or_unset_uses_stored_budget(self, monkeypatch, configured):
+        from blueprints.ynab.sync import _current_state
+
+        monkeypatch.setenv("YNAB_BUDGET_ID", configured)
+        await self._state("uuid-a")
+        assert (await _current_state()).budget_id == "uuid-a"
+
+    async def test_different_configured_budget_is_an_error(self, monkeypatch):
+        from blueprints.ynab.sync import _current_state
+
+        monkeypatch.setenv("YNAB_BUDGET_ID", "uuid-b")
+        await self._state("uuid-a")
+        with pytest.raises(ValueError, match="clear the ynab_"):
+            await _current_state()
+
+    async def test_multiple_stored_budgets_is_an_error(self, monkeypatch):
+        from blueprints.ynab.sync import _current_state
+
+        monkeypatch.delenv("YNAB_BUDGET_ID", raising=False)
+        await self._state("uuid-a")
+        await self._state("uuid-b")
+        with pytest.raises(ValueError):
+            await _current_state()
+
+
+async def test_parallel_syncs_share_one_failure(db, monkeypatch):
+    from blueprints.ynab import sync
+
+    calls = []
+
+    class FailingYNAB:
+        async def fetch_budget(self, budget_id, since):
+            calls.append(budget_id)
+            raise TimeoutError("ynab down")
+
+    monkeypatch.setattr(sync, "YNABService", FailingYNAB)
+    monkeypatch.setattr(sync, "_last_failure", float("-inf"))
+    await asyncio.gather(
+        *(sync.ensure_fresh() for _ in range(3)), return_exceptions=True
+    )
+    assert len(calls) == 1