Add job processing system with streaming endpoint

Introduced a `processing` router to handle job streaming using server-sent events. Added `Jobs` and `JobStatus` models for managing job-related data, along with database creation logic. Updated the `sample` router to create new job entries during experiment creation.

backend/app/routers/__init__.py

@@ -5,6 +5,7 @@ from .proposal import router as proposal_router
 from .dewar import dewar_router
 from .shipment import shipment_router
 from .auth import router as auth_router
+from .processing import router as processing_router
 from .protected_router import protected_router as protected_router
 
 __all__ = [
@@ -15,5 +16,6 @@ __all__ = [
     "dewar_router",
     "shipment_router",
     "auth_router",
+    "processing_router",
     "protected_router",
 ]