feat: add theoretical balance history to team overview stats

Reconstructs each active player's balance per month (same backward
technique as the existing cash-balance history) so the overview stats
endpoint can report what the team balance would be if all currently
open dues had already been paid.

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

myteamwallet_backend/src/teams/teams.service.spec.ts

@@ -0,0 +1,127 @@
+import { TeamsService } from './teams.service';
+import { DEACTIVATION_ADJUSTMENT_NOTE_PREFIX } from './team-members.service';
+
+function monthKey(monthsAgo: number): string {
+  const now = new Date();
+  const d = new Date(now.getFullYear(), now.getMonth() - monthsAgo, 1);
+  return `${d.getFullYear()}-${String(d.getMonth() + 1).padStart(2, '0')}`;
+}
+
+function isoDate(monthsAgo: number, day: number): string {
+  const now = new Date();
+  return new Date(now.getFullYear(), now.getMonth() - monthsAgo, day).toISOString();
+}
+
+describe('TeamsService#getOverviewStats theoretical balance', () => {
+  const repository = { findOneOrFail: jest.fn() };
+  const access = { assertMember: jest.fn() };
+  let service: TeamsService;
+
+  beforeEach(() => {
+    jest.resetAllMocks();
+    access.assertMember.mockResolvedValue(undefined);
+    service = new TeamsService(
+      repository as any,
+      {} as any,
+      {} as any,
+      {} as any,
+      {} as any,
+      {} as any,
+      { info: jest.fn(), debug: jest.fn(), warn: jest.fn() } as any,
+      access as any,
+    );
+  });
+
+  it('adds still-open, unpaid debt to the theoretical balance while leaving the actual cash balance untouched', async () => {
+    repository.findOneOrFail.mockResolvedValue({
+      id: 9,
+      balance: 100,
+      // Bewegung liegt außerhalb des 12-Monats-Fensters, damit der Ist-Kassenstand
+      // über das gesamte sichtbare Fenster flach bei 100 bleibt.
+      transactions: [{ date: isoDate(13, 5), amount: 100, type: { name: 'credit' } }],
+      players: [
+        {
+          id: 1,
+          firstName: 'Alex',
+          lastName: 'Muster',
+          active: true,
+          balance: -30,
+          transactions: [
+            {
+              date: isoDate(2, 10),
+              amount: 30,
+              type: { id: 11, name: 'fine' },
+              note: 'Zu spät zum Training',
+            },
+          ],
+        },
+      ],
+    });
+
+    const result = await service.getOverviewStats(9, 42);
+
+    expect(access.assertMember).toHaveBeenCalledWith(42, 9);
+    const beforeFine = result.balanceHistory.find((p) => p.month === monthKey(4));
+    const now = result.balanceHistory.find((p) => p.month === monthKey(0));
+
+    expect(beforeFine?.balance).toBe(100);
+    expect(beforeFine?.theoreticalBalance).toBe(100);
+    expect(now?.balance).toBe(100);
+    // Sanity-Check: entspricht team.balance (100) + aktuelle offene Beiträge (30).
+    expect(now?.theoreticalBalance).toBe(130);
+  });
+
+  it('excludes deactivation-adjustment transactions from the historical reconstruction', async () => {
+    repository.findOneOrFail.mockResolvedValue({
+      id: 9,
+      balance: 50,
+      transactions: [{ date: isoDate(13, 5), amount: 50, type: { name: 'credit' } }],
+      players: [
+        {
+          id: 1,
+          firstName: 'Alex',
+          lastName: 'Muster',
+          active: true,
+          balance: -20,
+          transactions: [
+            {
+              date: isoDate(6, 5),
+              amount: 999,
+              type: { id: 1, name: 'credit' },
+              note: `${DEACTIVATION_ADJUSTMENT_NOTE_PREFIX} #1`,
+            },
+            {
+              date: isoDate(1, 10),
+              amount: 20,
+              type: { id: 11, name: 'fine' },
+              note: 'Zu spät',
+            },
+          ],
+        },
+      ],
+    });
+
+    const result = await service.getOverviewStats(9, 42);
+
+    const beforeFine = result.balanceHistory.find((p) => p.month === monthKey(4));
+    const now = result.balanceHistory.find((p) => p.month === monthKey(0));
+
+    // Wäre die Ausgleichsbuchung (999) nicht ausgeschlossen, würde sie hier bereits
+    // durchschlagen (beforeFine liegt chronologisch nach ihrem Datum) — tut sie aber nicht.
+    expect(beforeFine?.theoreticalBalance).toBe(50);
+    expect(now?.theoreticalBalance).toBe(70);
+  });
+
+  it('keeps returning an empty balance history when the team has no cash movement at all', async () => {
+    repository.findOneOrFail.mockResolvedValue({
+      id: 9,
+      balance: 0,
+      transactions: [],
+      players: [{ id: 1, firstName: 'Alex', lastName: 'Muster', active: true, balance: 0, transactions: [] }],
+    });
+
+    const result = await service.getOverviewStats(9, 42);
+
+    expect(result.balanceHistory).toEqual([]);
+  });
+});