T3: Decimal for money — all monetary fields migrated from float

- All domain monetary fields now use Decimal:
  - Account.balance, Asset.value, Asset.growth_rate
  - Liability.balance, Liability.interest, Liability.payment
  - Transaction.amount, RecurringCashflow.amount
  - ExchangeRate.rate
  - ForecastScenario.income_multiplier, expense_multiplier, growth_multiplier
- CurrencyConverter: all arithmetic in Decimal, quantize to 0.01 with ROUND_HALF_UP
- ForecastService: Decimal arithmetic throughout (income, expenses, balance, growth, liability cost)
- ScenarioService: Decimal multipliers, deepcopy safe with Decimal fields
- assistant.py: _DecimalEncoder for json.dumps (Decimal -> str in JSON)

Pydantic v2 + Decimal:
- model_dump(mode='json') converts Decimal to str (JSON-safe)
- model_validate() parses str back to Decimal
- Round-trip preserves precision (100.50 stays 100.50)

Tests: 63/63 pass.

application/scenarios.py

@@ -1,4 +1,5 @@
 from copy import deepcopy
+from decimal import Decimal
 
 from domain import FinancialModel, ForecastScenario
 from application.forecast import ForecastService
@@ -11,23 +12,23 @@ class ScenarioError(Exception):
 DEFAULT_SCENARIOS = {
     "baseline": ForecastScenario(
         name="baseline",
-        income_multiplier=1.0,
-        expense_multiplier=1.0,
-        growth_multiplier=1.0,
+        income_multiplier=Decimal("1.0"),
+        expense_multiplier=Decimal("1.0"),
+        growth_multiplier=Decimal("1.0"),
         description="Базовый сценарий без изменений",
     ),
     "optimistic": ForecastScenario(
         name="optimistic",
-        income_multiplier=1.15,
-        expense_multiplier=0.95,
-        growth_multiplier=1.2,
+        income_multiplier=Decimal("1.15"),
+        expense_multiplier=Decimal("0.95"),
+        growth_multiplier=Decimal("1.2"),
         description="Оптимистичный: доход +15%, расход -5%, рост активов +20%",
     ),
     "pessimistic": ForecastScenario(
         name="pessimistic",
-        income_multiplier=0.85,
-        expense_multiplier=1.1,
-        growth_multiplier=0.8,
+        income_multiplier=Decimal("0.85"),
+        expense_multiplier=Decimal("1.1"),
+        growth_multiplier=Decimal("0.8"),
         description="Пессимистичный: доход -15%, расход +10%, рост активов -20%",
     ),
 }
@@ -42,18 +43,18 @@ class ScenarioService:
 
         for t in model.transactions:
             if t.amount > 0:
-                t.amount *= scenario.income_multiplier
+                t.amount = t.amount * scenario.income_multiplier
             else:
-                t.amount *= scenario.expense_multiplier
+                t.amount = t.amount * scenario.expense_multiplier
 
         for r in model.recurring:
             if r.category == "income":
-                r.amount *= scenario.income_multiplier
+                r.amount = r.amount * scenario.income_multiplier
             else:
-                r.amount *= scenario.expense_multiplier
+                r.amount = r.amount * scenario.expense_multiplier
 
         for a in model.assets:
-            a.growth_rate *= scenario.growth_multiplier
+            a.growth_rate = a.growth_rate * scenario.growth_multiplier
 
         service = ForecastService(model)
         result = service.summary(months)
@@ -69,16 +70,19 @@ class ScenarioService:
 
     def what_if(
         self,
-        income_mult: float = 1.0,
-        expense_mult: float = 1.0,
-        growth_mult: float = 1.0,
+        income_mult: Decimal | float = Decimal("1.0"),
+        expense_mult: Decimal | float = Decimal("1.0"),
+        growth_mult: Decimal | float = Decimal("1.0"),
         months: int = 12,
     ) -> dict:
+        def to_d(v: Decimal | float) -> Decimal:
+            return v if isinstance(v, Decimal) else Decimal(str(v))
+
         scenario = ForecastScenario(
             name="what-if",
-            income_multiplier=income_mult,
-            expense_multiplier=expense_mult,
-            growth_multiplier=growth_mult,
+            income_multiplier=to_d(income_mult),
+            expense_multiplier=to_d(expense_mult),
+            growth_multiplier=to_d(growth_mult),
             description=(
                 f"What-if: income x{income_mult}, "
                 f"expense x{expense_mult}, growth x{growth_mult}"