refactor(scheduling): take a cron lease in one conditional upsert

acquire() made sure a row existed and then raced on a conditional UPDATE,
two statements the registrar wrapped in a transaction of its own. lint:tx
cannot follow a repository reached through a local variable, so it
reported the method. One INSERT ... ON CONFLICT DO UPDATE ... WHERE now
takes a free lease, renews the holder's and leaves anyone else's alone,
atomically on SQLite and on Postgres, and needs no transaction.

server/tests/unit/db/repositories/SchedulerLeases.repository.test.ts

@@ -6,6 +6,7 @@
 import { afterAll, beforeAll, beforeEach, describe, expect, it } from 'vitest';
 import { createSnapshotTestDb } from '../../../helpers/db-mock';
 import { createTestOrm, type TestOrm } from '../../../helpers/test-orm';
+import { deleteRows } from '../../../helpers/factories/rows';
 import { SchedulerLeases } from '../../../../src/db/entities/SchedulerLeases.entity';
 import type { SchedulerLeasesRepository } from '../../../../src/db/repositories/SchedulerLeases.repository';
 
@@ -17,8 +18,8 @@ beforeAll(async () => {
   t = await createTestOrm(testDb);
   leases = t.repo(SchedulerLeases);
 });
-beforeEach(() => {
-  testDb.prepare('DELETE FROM scheduler_leases').run();
+beforeEach(async () => {
+  await deleteRows(t, SchedulerLeases);
   t.clear();
 });
 afterAll(async () => {