fix(exams): add missing columns for AI grading

exam_questions was missing points, correct_answer, and grading_rubric
columns that gradeExamWithAI queries. Without them the select fails and
grading silently returns "Exam not found". Also adds evaluated_at to
exam_submissions for the save_exam_feedback RPC.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: guillermoscript

supabase/migrations/20260406193912_add_exam_grading_columns.sql

@@ -0,0 +1,23 @@
+-- =============================================================================
+-- Add missing columns to exam_questions for AI grading
+--
+-- The gradeExamWithAI server action selects points, correct_answer, and
+-- grading_rubric but these columns don't exist yet. Without them the query
+-- fails and AI grading silently returns "Exam not found".
+-- =============================================================================
+
+-- Points per question (default 10)
+ALTER TABLE public.exam_questions
+  ADD COLUMN IF NOT EXISTS points INTEGER DEFAULT 10;
+
+-- Correct answer text for true/false and free-text reference answers
+ALTER TABLE public.exam_questions
+  ADD COLUMN IF NOT EXISTS correct_answer TEXT;
+
+-- Rubric for manual/AI grading of free-text questions
+ALTER TABLE public.exam_questions
+  ADD COLUMN IF NOT EXISTS grading_rubric TEXT;
+
+-- Also add evaluated_at to exam_submissions if missing (used by save_exam_feedback RPC)
+ALTER TABLE public.exam_submissions
+  ADD COLUMN IF NOT EXISTS evaluated_at TIMESTAMPTZ;