feat: Datenmodell (SQLAlchemy) und Alembic-Migration

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

finance/tests/conftest.py

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+import pytest
+from sqlalchemy import create_engine
+from sqlalchemy.orm import Session
+from sqlalchemy.pool import StaticPool
+
+from app.db import Base
+
+
+@pytest.fixture
+def db():
+    engine = create_engine(
+        "sqlite://", poolclass=StaticPool,
+        connect_args={"check_same_thread": False},
+    )
+    Base.metadata.create_all(engine)
+    with Session(engine) as session:
+        yield session