refactor(tests): use client fixture for test isolation

Replace all AsyncClient instantiations in test_sources.py and test_jobs.py
with the shared  fixture from conftest.py. This ensures proper test
database isolation via dependency overrides.

- Remove imports of httpx AsyncClient, ASGITransport, and app
- Add client fixture parameter to all test functions
- Remove async with blocks around HTTP requests
- test_engine.py already uses db fixture correctly, no changes needed

backend/app/main.py

@@ -3,6 +3,7 @@ from fastapi import FastAPI
 from fastapi.middleware.cors import CORSMiddleware
 from contextlib import asynccontextmanager
 from app.database import engine, Base
+from app import models  # noqa: F401 - registers models with Base.metadata
 from app.routers import sources, jobs, executions, backups, settings, dashboard
 
 @asynccontextmanager