Establish a GradingOrchestrator service layer

[AishaA-cpu]

Problem

The current routing logic invokes grading tasks directly via Celery, without abstraction. As logic for grading, validation, or future priority/queue management grows, this direct coupling will make it hard to extend or optimize.

Proposal

• Implement a GradingOrchestrator service class responsible for all grading submission operations
• The orchestrator validates submission state, manages queueing (priority, retry, error cases), and decouples routers from background task mechanics
• Refactor routers to depend on this service (with FastAPI Depends)
• Add tests for orchestrator logic

Rationale

• Abstraction makes the grading workflow testable and ready for evolving requirements
• Enables async/priority queueing, alternate task backends, or business rules with minimal router changes

Acceptance Criteria

• Routers depend only on service interface, not Celery tasks directly
• GradingOrchestrator is covered by unit tests
• Submission flows are unchanged from user POV

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Labels: architecture, enhancement, backend