T2: Pydantic v2 — replace @dataclass with BaseModel

- All domain models now use pydantic.BaseModel
- Removed manual to_dict()/from_dict() — use model_dump(mode='json')/model_validate()
- Added field validators:
  - Account.balance >= 0
  - Transaction.amount != 0
  - RecurringCashflow.amount != 0, frequency in {daily, weekly, monthly, yearly}
  - Liability.interest, payment >= 0
  - ExchangeRate.rate > 0
  - ForecastScenario.multipliers >= 0
- pydantic v2 native UUID handling (auto str in JSON)
- Updated excel_sync.py to use model_validate()
- Updated tests/test_model.py and tests/test_currency.py
- Added pydantic>=2.0 to pyproject.toml

Tests: 63/63 pass.

domain/currency.py

@@ -1,4 +1,4 @@
-from dataclasses import dataclass
+from pydantic import BaseModel, Field, field_validator
 
 CURRENCY_SYMBOLS = {
     "RUB": "₽",
@@ -12,26 +12,17 @@ CURRENCY_SYMBOLS = {
 }
 
 
-@dataclass
-class ExchangeRate:
+class ExchangeRate(BaseModel):
     from_currency: str = "USD"
     to_currency: str = "RUB"
     rate: float = 80.0
 
-    def to_dict(self) -> dict:
-        return {
-            "from_currency": self.from_currency,
-            "to_currency": self.to_currency,
-            "rate": self.rate,
-        }
-
+    @field_validator("rate")
     @classmethod
-    def from_dict(cls, data: dict) -> "ExchangeRate":
-        return cls(
-            from_currency=data.get("from_currency", "USD"),
-            to_currency=data.get("to_currency", "RUB"),
-            rate=data.get("rate", 80.0),
-        )
+    def _rate_positive(cls, v: float) -> float:
+        if v <= 0:
+            raise ValueError("rate must be positive")
+        return v
 
 
 DEFAULT_RATES: list[ExchangeRate] = [