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/scenario.py

@@ -0,0 +1,82 @@
+from dataclasses import dataclass
+from datetime import date
+from decimal import ROUND_HALF_UP, Decimal
+
+from app.engine.loans import Installment
+from app.engine.recurrence import occurrences
+
+CENT = Decimal("0.01")
+
+
+@dataclass(frozen=True)
+class PlainRecurring:
+    id: int
+    name: str
+    amount: Decimal
+    rhythm: str
+    due_day: int
+    start_date: date | None
+    end_date: date | None
+    category_id: int | None
+
+
+@dataclass(frozen=True)
+class PlainPlanned:
+    name: str
+    amount: Decimal
+    due: date
+    category_id: int | None
+
+
+@dataclass(frozen=True)
+class PlainModifier:
+    target_type: str
+    target_id: int
+    kind: str
+    value: Decimal
+
+
+def _modified(amount: Decimal, category_id: int | None, recurring_id: int | None,
+              modifiers: list[PlainModifier]) -> Decimal | None:
+    for m in modifiers:
+        hit = ((m.target_type == "category" and category_id == m.target_id)
+               or (m.target_type == "recurring" and recurring_id == m.target_id))
+        if not hit:
+            continue
+        if m.kind == "remove":
+            return None
+        if m.kind == "percent":
+            amount = (amount * (Decimal(100) - m.value) / Decimal(100)).quantize(CENT, ROUND_HALF_UP)
+        elif m.kind == "absolute":
+            if amount < 0:
+                amount = min(Decimal("0"), amount + m.value)
+            else:
+                amount = max(Decimal("0"), amount - m.value)
+    return amount
+
+
+def build_cashflows(recurring: list[PlainRecurring], planned: list[PlainPlanned],
+                    loan_schedules: list[list[Installment]],
+                    loan_payouts: list[tuple[date, Decimal]],
+                    modifiers: list[PlainModifier],
+                    window_start: date, window_end: date) -> list[tuple[date, Decimal]]:
+    flows: list[tuple[date, Decimal]] = []
+    for r in recurring:
+        amount = _modified(r.amount, r.category_id, r.id, modifiers)
+        if amount is None:
+            continue
+        for d in occurrences(r.rhythm, r.due_day, window_start, window_end,
+                             r.start_date, r.end_date):
+            flows.append((d, amount))
+    for p in planned:
+        amount = _modified(p.amount, p.category_id, None, modifiers)
+        if amount is not None and window_start <= p.due <= window_end:
+            flows.append((p.due, amount))
+    for d, payout in loan_payouts:
+        if window_start <= d <= window_end:
+            flows.append((d, payout))
+    for sched in loan_schedules:
+        for inst in sched:
+            if window_start <= inst.due <= window_end:
+                flows.append((inst.due, inst.payment))
+    return sorted(flows)