feat: Datenmodell (SQLAlchemy) und Alembic-Migration

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

finance/tests/test_models.py

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+from datetime import date
+from decimal import Decimal
+
+from app.models.tables import Account, Statement, Transaction
+
+
+def test_account_transaction_roundtrip(db):
+    acc = Account(bank="DKB", iban="DE02120300000000202051", name="Giro", type="giro")
+    db.add(acc)
+    db.flush()
+    st = Statement(filename="a.pdf", bank="DKB", account_id=acc.id,
+                   period_start=date(2026, 6, 1), period_end=date(2026, 6, 30),
+                   opening_balance=Decimal("100.00"), closing_balance=Decimal("50.00"),
+                   status="draft")
+    db.add(st)
+    db.flush()
+    tx = Transaction(account_id=acc.id, statement_id=st.id,
+                     booking_date=date(2026, 6, 3), value_date=date(2026, 6, 3),
+                     amount=Decimal("-50.00"), purpose="Miete Juni",
+                     counterparty="Vermieter GmbH", status="draft",
+                     dedup_hash="abc", is_duplicate=False)
+    db.add(tx)
+    db.commit()
+    assert db.query(Transaction).one().amount == Decimal("-50.00")