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/recurring.py

@@ -1,33 +1,26 @@
-from dataclasses import dataclass, field
 from uuid import UUID, uuid4
 
+from pydantic import BaseModel, Field, field_validator
 
-@dataclass
-class RecurringCashflow:
-    id: UUID = field(default_factory=uuid4)
+
+class RecurringCashflow(BaseModel):
+    id: UUID = Field(default_factory=uuid4)
     start_date: str = ""
     end_date: str = ""
     frequency: str = "monthly"
     amount: float = 0.0
     category: str = ""
 
-    def to_dict(self) -> dict:
-        return {
-            "id": str(self.id),
-            "start_date": self.start_date,
-            "end_date": self.end_date,
-            "frequency": self.frequency,
-            "amount": self.amount,
-            "category": self.category,
-        }
-
+    @field_validator("frequency")
     @classmethod
-    def from_dict(cls, data: dict) -> "RecurringCashflow":
-        return cls(
-            id=UUID(data["id"]),
-            start_date=data.get("start_date", ""),
-            end_date=data.get("end_date", ""),
-            frequency=data.get("frequency", "monthly"),
-            amount=data.get("amount", 0.0),
-            category=data.get("category", ""),
-        )
+    def _frequency_known(cls, v: str) -> str:
+        if v not in {"daily", "weekly", "monthly", "yearly"}:
+            raise ValueError(f"unknown frequency: {v}")
+        return v
+
+    @field_validator("amount")
+    @classmethod
+    def _amount_nonzero(cls, v: float) -> float:
+        if v == 0:
+            raise ValueError("amount must be non-zero")
+        return v