feat: setup FastAPI main app with routers and test fixtures

- Main FastAPI app with all routers included
- CORS middleware for frontend
- Test fixtures with in-memory SQLite database
- Placeholder routers for future tasks

backend/app/routers/executions.py

@@ -0,0 +1,4 @@
+from fastapi import APIRouter
+
+router = APIRouter(prefix="/api/executions", tags=["executions"])
+# Will be implemented in later tasks