E2605: Complete backend support for specialized rubrics by topic

[PositivelyBookish]

Summary

This PR finalizes backend support for specialized review rubrics by topic.

It enables assignments to use a default review rubric while allowing individual topics to override that rubric, with support for round-specific behavior when rubrics vary by both topic and round.

Changes

• support topic-aware rubric mappings in assignment questionnaires
• add backend handling for retrieving topic-specific rubric data for an assignment
• add backend handling for saving topic-specific rubric selections
• preserve assignment-level review rubrics as the default fallback when no topic-specific override exists
• support rubric resolution for all required cases:
  • no variation by round or topic
  • variation by round only
  • variation by topic only
  • variation by both round and topic
• align assignment update handling with the persisted vary-by-topic / vary-by-round assignment state

Behavior

After this change:

• an assignment can keep a default review rubric
• each topic can optionally override that review rubric
• if rubrics vary by round, topic-specific rubric selection can also vary by round
• when no topic-specific rubric is defined, the assignment default is used

Testing

Verified through manual integration testing with the assignment editor by:

• enabling Review rubric varies by topic?
• selecting topic-specific review rubrics
• saving the assignment
• reloading the assignment editor
• confirming the selected rubric persisted for the topic

[github-actions]

	2 Warnings
⚠️	Pull request is too big (more than 500 LoC).
⚠️	RSpec tests seem shallow (single it blocks or no context). Consider improving test structure.

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