T1: restructure to domain/application/infrastructure + schema version 1

- Move cashflow_model/ -> domain/
- Move engine/ -> application/
- Move cli/, sync/, ai/ -> infrastructure/
- Update all imports across codebase and tests
- Add version=1 to FinancialModel.to_dict() with migration hook in from_dict
- Update pyproject.toml: cf entry point and packages.find

Tests: 63/63 pass.

domain/transaction.py

@@ -0,0 +1,33 @@
+from dataclasses import dataclass, field
+from uuid import UUID, uuid4
+
+
+@dataclass
+class Transaction:
+    id: UUID = field(default_factory=uuid4)
+    date: str = ""
+    account: str = ""
+    category: str = ""
+    amount: float = 0.0
+    description: str = ""
+
+    def to_dict(self) -> dict:
+        return {
+            "id": str(self.id),
+            "date": self.date,
+            "account": self.account,
+            "category": self.category,
+            "amount": self.amount,
+            "description": self.description,
+        }
+
+    @classmethod
+    def from_dict(cls, data: dict) -> "Transaction":
+        return cls(
+            id=UUID(data["id"]),
+            date=data["date"],
+            account=data.get("account", ""),
+            category=data.get("category", ""),
+            amount=data.get("amount", 0.0),
+            description=data.get("description", ""),
+        )