feat(exam): Phase 5 — исправление subtopic-фолбэков (058_exam_topics_textbook_fix.sql)

alg-equations: §10(drobno) -> algebra-8-ch2 §8 (дискриминант, массовый тип)
alg-inequalities: §13 -> algebra-8-ch3 §17 (квадратные/метод интервалов)
alg-polynomials: algebra-9 -> algebra-7-ch2 §14 (разложение, основной тип)
alg-numbers: algebra-9 hub -> math-6-ch4 (рациональные числа)
alg-arithmetic: algebra-9 hub -> math-6-ch1 (десятичные дроби)
alg-powers: algebra-9 hub -> algebra-7-ch1 §1 (натур. показатель)
alg-word-problems: algebra-9 hub -> math-6-ch2 (проценты, массовый тип)
geom-quadrilaterals: geometry-9-ch2 §9 -> geometry-8-ch1 §4 (параллелограмм)
geom-circle: geometry-9-ch2 §7 -> geometry-8-ch4 §8 (центральный/вписанный угол)

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

backend/src/db/migrations/058_exam_topics_textbook_fix.sql

@@ -0,0 +1,73 @@
+-- ═══════════════════════════════════════════════════════════════
+-- 058: Fix exam_topics textbook_slug/paragraph (subtopic-level fallback)
+--
+-- Migration 028 set coarse fallback values. This migration corrects them
+-- to align with the detailed per-subtopic maps from PLAN.md.
+-- These values act as fallback when a task has no task-level textbook_slug
+-- (e.g. theory-statements or newly added tasks not yet classified).
+-- ═══════════════════════════════════════════════════════════════
+
+-- ── Algebra subtopics (fix / improve) ──────────────────────────
+
+-- alg-numbers: rational numbers are in math-6-ch4 (better than algebra-9 hub)
+UPDATE exam_topics SET textbook_slug = 'math-6-ch4', textbook_paragraph = NULL
+ WHERE slug = 'alg-numbers' AND exam_key = 'math9';
+
+-- alg-arithmetic: basic arithmetic → math-6-ch1 (decimals, most common 5-6 topic)
+UPDATE exam_topics SET textbook_slug = 'math-6-ch1', textbook_paragraph = NULL
+ WHERE slug = 'alg-arithmetic' AND exam_key = 'math9';
+
+-- alg-powers: primary is natural exponent → algebra-7-ch1 §1
+UPDATE exam_topics SET textbook_slug = 'algebra-7-ch1', textbook_paragraph = 1
+ WHERE slug = 'alg-powers' AND exam_key = 'math9';
+
+-- alg-expressions: primary → algebra-9-ch1 §5 (rational expression transform)
+-- (028 already set this correctly)
+
+-- alg-polynomials: primary → algebra-7-ch2 §14 (factoring, most tested)
+UPDATE exam_topics SET textbook_slug = 'algebra-7-ch2', textbook_paragraph = 14
+ WHERE slug = 'alg-polynomials' AND exam_key = 'math9';
+
+-- alg-fractions: primary → algebra-9-ch1 §3 (add/subtract rational fractions)
+-- (028 already set §3; keep it)
+
+-- alg-equations: primary → algebra-8-ch2 §8 (quadratic by discriminant, most common)
+-- (028 had §10 from algebra-9-ch3 which is drobno-rational — fix to quadratic)
+UPDATE exam_topics SET textbook_slug = 'algebra-8-ch2', textbook_paragraph = 8
+ WHERE slug = 'alg-equations' AND exam_key = 'math9';
+
+-- alg-inequalities: primary → algebra-8-ch3 §17 (quadratic inequalities, method of intervals)
+-- (028 already set §13; fix to §17 which is the most common exam topic)
+UPDATE exam_topics SET textbook_slug = 'algebra-8-ch3', textbook_paragraph = 17
+ WHERE slug = 'alg-inequalities' AND exam_key = 'math9';
+
+-- alg-functions: primary → algebra-9-ch2 §6 (function of numeric argument)
+-- (028 already set §6; keep it)
+
+-- alg-progressions: primary → algebra-9-ch4 §15 (arithmetic progression)
+-- (028 already set §15; keep it)
+
+-- alg-word-problems: primary → math-6-ch2 (percentages — most common type)
+UPDATE exam_topics SET textbook_slug = 'math-6-ch2', textbook_paragraph = NULL
+ WHERE slug = 'alg-word-problems' AND exam_key = 'math9';
+
+-- ── Geometry subtopics (fix / improve) ──────────────────────────
+
+-- geom-triangles: primary → geometry-9-ch1 §1 (trig in right triangle, most tested)
+-- (028 had §1 of geometry-9-ch1; keep it)
+
+-- geom-quadrilaterals: primary → geometry-8-ch1 §4 (parallelogram, most common)
+-- (028 had §9 of geometry-9-ch2 — too specific; fix to general parallelogram)
+UPDATE exam_topics SET textbook_slug = 'geometry-8-ch1', textbook_paragraph = 4
+ WHERE slug = 'geom-quadrilaterals' AND exam_key = 'math9';
+
+-- geom-circle: primary → geometry-8-ch4 §8 (central angle / inscribed angle)
+-- (028 had §7 of geometry-9-ch2; fix to geometry-8-ch4 §8)
+UPDATE exam_topics SET textbook_slug = 'geometry-8-ch4', textbook_paragraph = 8
+ WHERE slug = 'geom-circle' AND exam_key = 'math9';
+
+-- geom-coordinates: primary → algebra-9-ch3 §12 (circle equation)
+-- (028 already set §12; keep it)
+
+-- theory-statements: leave NULL — no single textbook home
+-- (028 already left NULL; keep it)