fix: jobs router session handling, schedule creation, and tests

- Use new session in background task for thread safety
- Fix schedule creation to associate with job
- Fix schedule endpoint return type
- Update tests to use AsyncClient
- Add missing tests for list, get, delete, schedule
- Add updated_at field to Schedule model

backend/app/models.py

@@ -63,6 +63,11 @@ class Schedule(Base):
     cron_expression = Column(String, nullable=False)
     enabled = Column(Boolean, default=True)
     created_at = Column(DateTime, default=lambda: datetime.now(timezone.utc))
+    updated_at = Column(
+        DateTime,
+        default=lambda: datetime.now(timezone.utc),
+        onupdate=lambda: datetime.now(timezone.utc),
+    )
 
     job = relationship("Job", back_populates="schedule")