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/forecast.py

@@ -1,4 +1,5 @@
 from copy import deepcopy
+from decimal import Decimal, ROUND_HALF_UP
 
 from domain import Account, FinancialModel
 
@@ -7,6 +8,12 @@ class ForecastError(Exception):
     pass
 
 
+_ZERO = Decimal("0")
+_ONE = Decimal("1")
+_TWELVE = Decimal("12")
+_HUNDRED = Decimal("100")
+
+
 class ForecastService:
     def __init__(self, model: FinancialModel):
         self.model = deepcopy(model)
@@ -30,19 +37,19 @@ class ForecastService:
         # Asset growth — once per month, distributed across accounts proportionally
         for m in range(months):
             total_growth = sum(
-                a.value * a.growth_rate / 100 / 12
+                a.value * a.growth_rate / _HUNDRED / _TWELVE
                 for a in self.model.assets
             )
             month_rows = [r for r in results if r["month"] == m + 1]
-            total_bal = sum(r["balance"] for r in month_rows) or 1
+            total_bal = sum(r["balance"] for r in month_rows) or _ONE
             for r in month_rows:
                 share = r["balance"] / total_bal
-                r["income"] = round(r["income"] + total_growth * share, 2)
-                r["balance"] = round(r["balance"] + total_growth * share, 2)
+                r["income"] = _q(r["income"] + total_growth * share)
+                r["balance"] = _q(r["balance"] + total_growth * share)
 
             # Compound asset values for next month
             for a in self.model.assets:
-                a.value += a.value * a.growth_rate / 100 / 12
+                a.value = a.value + a.value * a.growth_rate / _HUNDRED / _TWELVE
 
         return results
 
@@ -50,8 +57,8 @@ class ForecastService:
         balance = account.balance
         monthly = []
         for m in range(months):
-            income = 0.0
-            expenses = 0.0
+            income = _ZERO
+            expenses = _ZERO
 
             for t in self.model.transactions:
                 if t.account == str(account.id):
@@ -68,23 +75,23 @@ class ForecastService:
 
             expenses += self._liability_cost(account)
 
-            balance += income - expenses
+            balance = balance + income - expenses
 
             monthly.append({
-                "balance": round(balance, 2),
-                "income": round(income, 2),
-                "expenses": round(expenses, 2),
+                "balance": _q(balance),
+                "income": _q(income),
+                "expenses": _q(expenses),
             })
         return monthly
 
-    def _liability_cost(self, account: Account) -> float:
-        total = 0.0
+    def _liability_cost(self, account: Account) -> Decimal:
+        total = _ZERO
         for liability in self.model.liabilities:
-            interest_cost = liability.balance * liability.interest / 100 / 12
+            interest_cost = liability.balance * liability.interest / _HUNDRED / _TWELVE
             total += interest_cost
-            liability.balance -= liability.payment - interest_cost
+            liability.balance = liability.balance - (liability.payment - interest_cost)
             if liability.balance < 0:
-                liability.balance = 0
+                liability.balance = _ZERO
         return total
 
     def summary(self, months: int = 12) -> dict:
@@ -97,7 +104,12 @@ class ForecastService:
         all_expenses = sum(r["expenses"] for r in results)
         return {
             "total_balance": final["balance"],
-            "total_income": round(all_income, 2),
-            "total_expenses": round(all_expenses, 2),
+            "total_income": _q(all_income),
+            "total_expenses": _q(all_expenses),
             "months": months,
         }
+
+
+def _q(v: Decimal) -> Decimal:
+    """Quantize to 2 decimal places, ROUND_HALF_UP."""
+    return v.quantize(Decimal("0.01"), rounding=ROUND_HALF_UP)