feat: add database models with cascade delete

Implements SQLAlchemy 2.0 async ORM models (Config, Subscription, ExportLog) with cascade delete-orphan relationships. Upgrades SQLAlchemy to 2.0.49 for Python 3.14 compatibility. Adds pytest conftest with in-memory SQLite fixture; all 4 TDD tests pass.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/tests/conftest.py

@@ -0,0 +1,18 @@
+import sys
+import os
+sys.path.insert(0, os.path.join(os.path.dirname(__file__), ".."))
+
+import pytest_asyncio
+from sqlalchemy.ext.asyncio import create_async_engine, async_sessionmaker
+from models import Base
+
+
+@pytest_asyncio.fixture
+async def db_session():
+    engine = create_async_engine("sqlite+aiosqlite:///:memory:")
+    async with engine.begin() as conn:
+        await conn.run_sync(Base.metadata.create_all)
+    Session = async_sessionmaker(engine, expire_on_commit=False)
+    async with Session() as session:
+        yield session
+    await engine.dispose()