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/recurring.py

@@ -0,0 +1,33 @@
+from dataclasses import dataclass, field
+from uuid import UUID, uuid4
+
+
+@dataclass
+class RecurringCashflow:
+    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,
+        }
+
+    @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", ""),
+        )