fix: strengthen deactivation-exclusion test and align sign rule with canonical logic

- teams.service.spec.ts: pick a checkpoint older than the adjustment's own
  month so the exclusion test actually fails without the exclusion filter
- teams.service.ts: only negate fine/levy/fee amounts when positive,
  matching TeamMembersService.recomputeBalance and Transaction.setBalance()
  exactly, instead of negating unconditionally
- teams.service.ts: outstanding-history helper now returns a positive
  value when players owe money, matching the house convention already
  established by getOverview()'s team.outstanding
- overview.spec.ts: assert the balance chart's legend becomes visible
- teams.service.spec.ts: add coverage for inactive-player exclusion and
  a positive-balance (prepaid credit) case

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

myteamwallet_frontend_modern/src/app/features/team/overview/overview.spec.ts

@@ -197,6 +197,7 @@ describe('Overview', () => {
     expect(balanceChart.data.datasets[0].data).toEqual([100, 125]);
     expect(balanceChart.data.datasets[1].label).toBe('Theoretisch (inkl. offene Beiträge)');
     expect(balanceChart.data.datasets[1].data).toEqual([100, 150]);
+    expect(balanceChart.options?.plugins?.legend?.position).toBe('bottom');
     expect(flowChart.type).toBe('bar');
     expect(flowChart.data.datasets).toHaveLength(2);
     expect(outstandingChart.type).toBe('bar');