fix(sandbox): fit the nightly cleanup inside PostgREST's 8s session cap (#1451)

* fix(sandbox): fit the nightly cleanup inside PostgREST's 8s session cap

Follow-up to #1449. Profiling the repaired teardown on prod puts one
sandbox user at ~3s (the auth.users delete fans out over ~250 FK triggers;
FK indexes were tried inside an aborted transaction and do not help), while
every PostgREST session inherits authenticator's statement_timeout = 8s.
The nightly RPC call therefore times out and ROLLS BACK wholesale: a second
silent-failure mode for the same cron.

- cleanup_expired_sandbox_users gets a function-local statement_timeout of
  290s (same sanctioned pattern as undo_sie_import, 20260702154500) via
  migration 20260807150000.
- The cron route bounds each night to 60 users (~180s), exports
  maxDuration = 300, and the backlog drains over a few nights.
- Tests: route asserts the bounded rpc call and the maxDuration budget; the
  pg suite pins proconfig containing statement_timeout=290s.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* fix(db): index journal_entry_lines.account_id (chart-account cascade seq-scans 730k rows)

Caught live during the backlog purge: DELETE FROM auth.users cascades chart_of_accounts deletion, whose ON DELETE SET NULL fires an unindexed UPDATE over journal_entry_lines per account (~37 per sandbox company). This is the bulk of the ~3s per-user teardown cost.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

tests/pg/sandbox-cleanup.pg.test.ts

@@ -352,6 +352,21 @@ describe('sandbox cleanup RPCs (pg)', () => {
     }
   })
 
+  it('cleanup_expired_sandbox_users carries its own statement_timeout', async () => {
+    // PostgREST sessions inherit authenticator's 8s statement_timeout while
+    // one sandbox teardown costs ~3s, so without a function-local override
+    // the nightly batch times out and rolls back wholesale (migration
+    // 20260807150000, same pattern as undo_sie_import).
+    const { rows } = await getPool().query<{ proconfig: string[] | null }>(
+      `SELECT p.proconfig
+       FROM pg_proc p
+       JOIN pg_namespace n ON n.oid = p.pronamespace
+       WHERE n.nspname = 'public' AND p.proname = 'cleanup_expired_sandbox_users'`,
+    )
+    expect(rows).toHaveLength(1)
+    expect(rows[0]!.proconfig ?? []).toContain('statement_timeout=290s')
+  })
+
   it('is executable by service_role only', async () => {
     const { rows } = await getPool().query<{
       svc_user: boolean