feat: Szenario-Engine (Projektion, Modifikatoren, Zahlungsstrom)

Implementiert Task 5 mit TDD-Ansatz: projection.py mit Projection-Klasse
und project()-Funktion für Cashflow-Simulation; scenario.py mit PlainRecurring,
PlainPlanned, PlainModifier Dataclasses und build_cashflows() für Szenarien-Zahlungsströme
mit Modifier-Support (remove, percent, absolute). Alle 15 Tests grün.

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

finance/tests/test_scenario_engine.py

@@ -0,0 +1,37 @@
+from datetime import date
+from decimal import Decimal
+
+from app.engine.loans import loan_schedule
+from app.engine.scenario import (PlainModifier, PlainPlanned, PlainRecurring,
+                                 build_cashflows)
+
+W = (date(2026, 8, 1), date(2026, 10, 31))
+
+
+def test_recurring_with_percent_cut():
+    rec = [PlainRecurring(id=1, name="Marketing", amount=Decimal("-1000"),
+                          rhythm="monthly", due_day=5, start_date=None,
+                          end_date=None, category_id=7)]
+    mods = [PlainModifier(target_type="category", target_id=7,
+                          kind="percent", value=Decimal("50"))]
+    flows = build_cashflows(rec, [], [], [], mods, *W)
+    assert flows == [(date(2026, 8, 5), Decimal("-500.00")),
+                     (date(2026, 9, 5), Decimal("-500.00")),
+                     (date(2026, 10, 5), Decimal("-500.00"))]
+
+
+def test_remove_modifier_and_planned_and_loan():
+    rec = [PlainRecurring(id=2, name="Abo", amount=Decimal("-50"),
+                          rhythm="monthly", due_day=1, start_date=None,
+                          end_date=None, category_id=None)]
+    mods = [PlainModifier(target_type="recurring", target_id=2,
+                          kind="remove", value=Decimal("0"))]
+    planned = [PlainPlanned(name="Steuer", amount=Decimal("-2000"),
+                            due=date(2026, 9, 15), category_id=None)]
+    sched = loan_schedule(Decimal("10000"), Decimal("6"), 48, date(2026, 8, 1), "annuity")
+    flows = build_cashflows(rec, planned, [sched],
+                            [(date(2026, 8, 1), Decimal("10000"))], mods, *W)
+    days = [d for d, _ in flows]
+    assert date(2026, 8, 1) in days            # Kredit-Auszahlung
+    assert date(2026, 9, 15) in days           # Einmalposten
+    assert all(a != Decimal("-50") for _, a in flows)  # Abo entfernt