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/app/engine/projection.py

@@ -0,0 +1,39 @@
+from collections import defaultdict
+from dataclasses import dataclass
+from datetime import date, timedelta
+from decimal import Decimal
+
+
+@dataclass(frozen=True)
+class Projection:
+    series: list[tuple[date, Decimal]]
+    low_point: tuple[date, Decimal]
+    first_below_zero: date | None
+    first_below_threshold: date | None
+
+
+def project(start_balance: Decimal, start_date: date,
+            cashflows: list[tuple[date, Decimal]], horizon_days: int,
+            threshold: Decimal = Decimal("0")) -> Projection:
+    end = start_date + timedelta(days=horizon_days)
+    by_day: dict[date, Decimal] = defaultdict(lambda: Decimal("0"))
+    for d, amount in cashflows:
+        if start_date < d <= end:
+            by_day[d] += amount
+    series: list[tuple[date, Decimal]] = []
+    balance = start_balance
+    low = (start_date, start_balance)
+    below0: date | None = None
+    below_t: date | None = None
+    d = start_date + timedelta(days=1)
+    while d <= end:
+        balance += by_day.get(d, Decimal("0"))
+        series.append((d, balance))
+        if balance < low[1]:
+            low = (d, balance)
+        if below0 is None and balance < 0:
+            below0 = d
+        if below_t is None and balance < threshold:
+            below_t = d
+        d += timedelta(days=1)
+    return Projection(series, low, below0, below_t)