Migrate all 29 money columns from float to NUMERIC/Decimal (BE-02)

Models use Money = Annotated[Decimal, PlainSerializer(float)] so storage
is exact NUMERIC(15,2) (rates 15,6) while JSON keeps emitting plain
numbers — the frontend contract is unchanged, pinned by
test_models_serialization.py. Service layers coerce to float at their
boundaries (projection math, rate multipliers, agent tool output, the
savings constants become Decimal) so no naive Decimal/float mixing can
raise. Migration 7f567c8bafdb applied to dev: per-row values and the
table sum byte-identical before/after; endpoints verified live on the
new schema. 59/59 tests green.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/tests/test_models_serialization.py

@@ -0,0 +1,67 @@
+"""Pin the Money serialization contract (Phase 2 Decimal migration, BE-02):
+NUMERIC storage, but JSON emits plain numbers so the frontend's `number`
+types keep working."""
+
+import json
+from datetime import datetime
+from decimal import Decimal
+
+from sqlmodel import select
+
+from app.models.models import (
+    Account,
+    AccountRead,
+    Bank,
+    Currency,
+    Transaction,
+    TransactionRead,
+)
+
+
+def test_money_columns_are_numeric():
+    assert str(Transaction.__table__.c.amount.type) == "NUMERIC(15, 2)"
+    assert str(Account.__table__.c.balance.type) == "NUMERIC(15, 2)"
+
+
+def test_money_serializes_as_json_number():
+    read = TransactionRead(
+        id=1,
+        amount=Decimal("25000.50"),
+        merchant="X",
+        date=datetime(2026, 4, 1),
+        created_at=datetime(2026, 4, 1),
+    )
+    dumped = read.model_dump(mode="json")
+    assert dumped["amount"] == 25000.5
+    assert isinstance(dumped["amount"], float)
+    # and the whole payload is json.dumps-able (no Decimal leaks)
+    json.dumps(dumped)
+
+
+def test_money_accepts_float_input():
+    # n8n and the SPA POST plain JSON numbers; they must coerce to Decimal.
+    read = AccountRead(
+        id=1,
+        bank=Bank.BAC,
+        currency=Currency.CRC,
+        label="x",
+        balance=1234.56,
+        updated_at=datetime(2026, 4, 1),
+    )
+    assert read.balance == Decimal("1234.56")
+
+
+def test_decimal_roundtrip_through_db(session):
+    session.add(
+        Transaction(
+            amount=Decimal("9999.99"),
+            merchant="ROUNDTRIP",
+            date=datetime(2026, 4, 1),
+        )
+    )
+    session.commit()
+    row = session.exec(
+        select(Transaction).where(Transaction.merchant == "ROUNDTRIP")
+    ).one()
+    assert isinstance(row.amount, Decimal)
+    assert row.amount == Decimal("9999.99")