feat(exam): Phase 2 — схема per-task textbook link (057_exam_task_textbook.sql)

ALTER TABLE exam_tasks ADD COLUMN textbook_slug TEXT;
ALTER TABLE exam_tasks ADD COLUMN textbook_paragraph INTEGER;

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/src/db/migrations/057_exam_task_textbook.sql

@@ -0,0 +1,14 @@
+-- ═══════════════════════════════════════════════════════════════
+-- 057: Per-task textbook link for exam_tasks
+--
+-- Adds two columns to exam_tasks:
+--   textbook_slug      — textbook chapter slug (e.g. 'algebra-8-ch2')
+--   textbook_paragraph — paragraph number inside that chapter (pN → N as integer)
+--
+-- These are populated by backend/scripts/tag-exam-textbook.js.
+-- The controller (exam-prep.js) prefers these task-level values
+-- over the subtopic-level fallback from exam_topics.
+-- ═══════════════════════════════════════════════════════════════
+
+ALTER TABLE exam_tasks ADD COLUMN textbook_slug TEXT;
+ALTER TABLE exam_tasks ADD COLUMN textbook_paragraph INTEGER;