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.

infrastructure/ai/assistant.py

@@ -1,4 +1,5 @@
 import json
+from decimal import Decimal
 
 from infrastructure.ai import prompts
 from domain import CurrencyConverter, FinancialModel
@@ -9,6 +10,15 @@ class AssistantError(Exception):
     pass
 
 
+class _DecimalEncoder(json.JSONEncoder):
+    """JSON-сериализатор: Decimal → str (для AI-промптов)."""
+
+    def default(self, o):
+        if isinstance(o, Decimal):
+            return str(o)
+        return super().default(o)
+
+
 class AssistantService:
     def __init__(
         self,
@@ -26,8 +36,8 @@ class AssistantService:
         summary = forecast_service.summary(months)
 
         prompt = prompts.format_context(
-            model_json=json.dumps(self.model.to_dict(), indent=2, ensure_ascii=False),
-            forecast_json=json.dumps(forecast_result, indent=2, ensure_ascii=False),
+            model_json=json.dumps(self.model.to_dict(), indent=2, ensure_ascii=False, cls=_DecimalEncoder),
+            forecast_json=json.dumps(forecast_result, indent=2, ensure_ascii=False, cls=_DecimalEncoder),
             months=months,
             base_currency=self.model.base_currency,
             display_currency=self.display_currency,
@@ -45,8 +55,8 @@ class AssistantService:
         forecast_result = forecast_service.forecast_cashflow(months)
 
         prompt = prompts.ADVICE_PROMPT.format(
-            model_json=json.dumps(self.model.to_dict(), indent=2, ensure_ascii=False),
-            forecast_json=json.dumps(forecast_result, indent=2, ensure_ascii=False),
+            model_json=json.dumps(self.model.to_dict(), indent=2, ensure_ascii=False, cls=_DecimalEncoder),
+            forecast_json=json.dumps(forecast_result, indent=2, ensure_ascii=False, cls=_DecimalEncoder),
             question=question,
             base_currency=self.model.base_currency,
             display_currency=self.display_currency,