Add AI grading statistics/feedback and refined AI comment persistence workflow

[tzj04]

Summary

This PR introduces the AI comment workflow backend and LLM usage tracking end-to-end.

What Changed

• Implemented AI comment generation controller flow.
• Added AI comment versioning and finalization support.
• Added comment selection support and word-level version tracking.
• Added LLM usage logs and LLM feedback schemas.
• Added LLM stats context and admin endpoints/views for monitoring.
• Integrated LLM stats into assessment-related flows.
• Refactored API by removing redundant final comment endpoint/column.
• Refactored versioning by removing diff column generation/storage.

Backend Scope

• Core AI comments logic:
  • lib/cadet/ai_comments.ex
  • lib/cadet/ai_comments/ai_comment.ex
  • lib/cadet/ai_comments/ai_comment_version.ex
• LLM stats/logging:
  • lib/cadet/llm_stats.ex
  • lib/cadet/llm_stats/llm_usage_log.ex
  • lib/cadet/llm_stats/llm_feedback.ex
• Controllers/routes/views:
  • lib/cadet_web/controllers/generate_ai_comments.ex
  • lib/cadet_web/admin_controllers/admin_llm_stats_controller.ex
  • lib/cadet_web/router.ex
• Migrations:
  • priv/repo/migrations/20260225120000_create_ai_comment_versions.exs
  • priv/repo/migrations/20260225120001_add_fields_to_ai_comments.exs
  • priv/repo/migrations/20260303120000_create_llm_usage_logs.exs
  • priv/repo/migrations/20260303120001_create_llm_feedback.exs
  • priv/repo/migrations/20260303120002_add_question_id_to_llm_feedback.exs
  • priv/repo/migrations/20260316120000_remove_final_comment_from_ai_comment_logs.exs
  • priv/repo/migrations/20260316123000_remove_diff_columns_from_ai_comment_versions.exs

[Copilot]

Pull request overview

Adds backend support for tracking LLM “Generate Comments” usage + collecting feedback, while updating the AI comments flow to store chosen comment indices and per-comment edit/version history. Also extends assessment aggregates to expose whether an assessment contains any LLM-enabled questions.

Changes:

• Introduces LLMStats context + schemas, with admin endpoints for per-assessment/per-question usage stats and feedback submission/retrieval.
• Refactors AI comment finalization to persist selected_indices, finalization metadata, and per-comment version entries (tutor edits).
• Extends assessment aggregates with has_llm_questions and exposes isLlmGraded in assessment JSON views.

Reviewed changes

Copilot reviewed 22 out of 22 changed files in this pull request and generated 13 comments.

Show a summary per file

File	Description
priv/repo/migrations/20260316123000_remove_diff_columns_from_ai_comment_versions.exs	Drops unused diff columns from ai_comment_versions.
priv/repo/migrations/20260316120000_remove_final_comment_from_ai_comment_logs.exs	Removes legacy final_comment column from ai_comment_logs.
priv/repo/migrations/20260303120002_add_question_id_to_llm_feedback.exs	Adds question_id FK + index to llm_feedback.
priv/repo/migrations/20260303120001_create_llm_feedback.exs	Creates llm_feedback table for user feedback.
priv/repo/migrations/20260303120000_create_llm_usage_logs.exs	Creates llm_usage_logs table for usage events.
priv/repo/migrations/20260225120001_add_fields_to_ai_comments.exs	Adds selected/finalization metadata fields to ai_comment_logs.
priv/repo/migrations/20260225120000_create_ai_comment_versions.exs	Creates ai_comment_versions table for per-comment edit history.
lib/cadet/llm_stats/llm_usage_log.ex	Ecto schema/changeset for usage log inserts.
lib/cadet/llm_stats/llm_feedback.ex	Ecto schema/changeset for feedback inserts.
lib/cadet/llm_stats.ex	Context queries for usage stats aggregation + feedback workflows.
lib/cadet/assessments/query.ex	Adds has_llm_questions aggregate via JSON prompt detection.
lib/cadet/assessments/assessment.ex	Adds virtual has_llm_questions field to assessment struct.
lib/cadet/ai_comments/ai_comment.ex	Updates AI comment schema to new finalization fields + versions association.
lib/cadet/ai_comments/ai_comment_version.ex	New schema/changeset for comment version rows.
lib/cadet/ai_comments.ex	Adds APIs to save selections and create/fetch comment versions.
lib/cadet_web/views/assessments_view.ex	Exposes isLlmGraded in student-facing assessment JSON.
lib/cadet_web/admin_views/admin_assessments_view.ex	Exposes isLlmGraded in admin assessment JSON.
lib/cadet_web/router.ex	Adds admin routes for LLM stats/feedback; updates save-chosen-comments route.
lib/cadet_web/controllers/generate_ai_comments.ex	Logs usage; replaces final-comment saving with chosen-indices + edits/versioning; updates swagger docs.
lib/cadet_web/admin_controllers/admin_llm_stats_controller.ex	New controller for LLM stats + feedback endpoints.
config/cadet.exs	Adds an example production configuration file.
.tool-versions	Adds Elixir/Erlang tool version pins.

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[RichDom2185]

In addition to the Copilot comments