feat: Kredit-Mathematik (Annuitaet, endfaellig)

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

finance/tests/test_loans.py

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+from datetime import date
+from decimal import Decimal
+
+from app.engine.loans import add_months, annuity_payment, loan_schedule
+
+
+def test_add_months_clamps_month_end():
+    assert add_months(date(2026, 1, 31), 1) == date(2026, 2, 28)
+
+
+def test_annuity_payment():
+    # 10.000 EUR, 6 % p.a., 48 Monate -> 234,85 EUR Monatsrate
+    assert annuity_payment(Decimal("10000"), Decimal("6"), 48) == Decimal("234.85")
+
+
+def test_annuity_schedule_amortizes_fully():
+    plan = loan_schedule(Decimal("10000"), Decimal("6"), 48, date(2026, 8, 1), "annuity")
+    assert len(plan) == 48
+    assert plan[-1].remaining == Decimal("0.00")
+    assert plan[0].due == date(2026, 9, 1)
+    assert plan[0].interest == Decimal("50.00")  # 10000 * 0,5 %
+    assert plan[0].payment < 0
+
+
+def test_bullet_loan():
+    plan = loan_schedule(Decimal("10000"), Decimal("6"), 12, date(2026, 8, 1), "bullet")
+    assert all(i.principal == 0 for i in plan[:-1])
+    assert plan[-1].principal == Decimal("10000")
+    assert plan[-1].remaining == Decimal("0.00")