feat: add database models and alembic setup

- SQLAlchemy async models for sources, jobs, schedules, executions, backups, settings
- Alembic migration configuration

backend/app/database.py

@@ -0,0 +1,16 @@
+from sqlalchemy.ext.asyncio import create_async_engine, AsyncSession, async_sessionmaker
+from sqlalchemy.orm import declarative_base
+
+DATABASE_URL = "sqlite+aiosqlite:///./backup_tool.db"
+
+engine = create_async_engine(DATABASE_URL, echo=True)
+AsyncSessionLocal = async_sessionmaker(engine, class_=AsyncSession, expire_on_commit=False)
+
+Base = declarative_base()
+
+async def get_db():
+    async with AsyncSessionLocal() as session:
+        try:
+            yield session
+        finally:
+            await session.close()