feat(exam9): link tasks to textbook + difficulty-ordered random + topic exclusion

Practice (random) now picks tasks by ascending difficulty so the first
slot is always level 1 and the session ramps up. Adds ?exclude= to drop
specific subtopics from the random pool, with a per-section checkbox
modal in the UI.

Each task carries a topic_ref (textbook chapter + paragraph) shown as
a 'Учить тему · §N' button next to the solution, deep-linking to the
right section of /textbook/<slug>. Mapping seeded for all 15 math9
subtopics in migration 028.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/css/exam-prep.css

@@ -1054,3 +1054,84 @@
   .tc-input-row { gap: 8px; }
   .tc-ans-input { max-width: 100%; }
 }
+
+/* ── "Учить тему" link inside a task card ───────────────────────── */
+.tc-sol-row {
+  display: flex; align-items: center; gap: 10px;
+  flex-wrap: wrap;
+}
+.tc-ref-btn {
+  display: inline-flex; align-items: center; gap: 6px;
+  margin-left: auto;
+  padding: 6px 12px;
+  border: 1.5px solid var(--violet, #7c3aed);
+  border-radius: 9px;
+  background: rgba(124, 58, 237, 0.08);
+  color: var(--violet, #7c3aed);
+  font-family: 'Manrope', sans-serif; font-size: .82rem; font-weight: 700;
+  text-decoration: none;
+  transition: background .15s, color .15s;
+}
+.tc-ref-btn:hover {
+  background: var(--violet, #7c3aed); color: #fff;
+}
+.tc-ref-btn svg { width: 14px; height: 14px; }
+
+/* ── Difficulty-progression note above the task list ─────────────── */
+.pr-difficulty-note {
+  display: flex; align-items: center; gap: 8px;
+  margin-top: 10px;
+  padding: 8px 12px;
+  background: rgba(6, 214, 160, 0.08);
+  border-left: 3px solid #06D6A0;
+  border-radius: 6px;
+  font-size: .82rem;
+  color: var(--text-2);
+}
+.pr-difficulty-note i, .pr-difficulty-note svg { width: 14px; height: 14px; flex-shrink: 0; }
+
+/* ── Exclude-topics button + count badge ─────────────────────────── */
+.pr-exclude-btn {
+  display: inline-flex; align-items: center; gap: 6px;
+  position: relative;
+}
+.pr-exclude-count {
+  display: inline-flex; align-items: center; justify-content: center;
+  min-width: 18px; height: 18px;
+  padding: 0 5px;
+  border-radius: 9px;
+  background: var(--violet, #7c3aed); color: #fff;
+  font-size: .7rem; font-weight: 800; line-height: 1;
+}
+
+/* ── Exclude-topics modal ────────────────────────────────────────── */
+.pr-ex-form { display: flex; flex-direction: column; gap: 16px; max-height: 60vh; overflow-y: auto; }
+.pr-ex-hint { font-size: .82rem; color: var(--text-3); margin: 0; }
+.pr-ex-section { border: 1px solid var(--border); border-radius: 10px; padding: 10px 12px; }
+.pr-ex-section-head {
+  display: flex; align-items: center; justify-content: space-between;
+  margin-bottom: 8px;
+  font-family: 'Unbounded', sans-serif; font-size: .82rem; font-weight: 800;
+  letter-spacing: .02em; color: var(--text);
+}
+.pr-ex-toggle-all {
+  border: 1px solid var(--border-h); background: transparent; color: var(--text-2);
+  padding: 3px 10px; border-radius: 6px;
+  font-family: 'Manrope', sans-serif; font-size: .72rem; font-weight: 700;
+  cursor: pointer; transition: all .12s;
+}
+.pr-ex-toggle-all:hover { border-color: var(--violet); color: var(--violet); }
+.pr-ex-items { display: flex; flex-direction: column; gap: 4px; }
+.pr-ex-item {
+  display: flex; align-items: center; gap: 10px;
+  padding: 6px 8px;
+  border-radius: 6px;
+  cursor: pointer;
+  font-size: .9rem;
+  transition: background .12s;
+}
+.pr-ex-item:hover { background: var(--border); }
+.pr-ex-item input { accent-color: var(--violet); flex-shrink: 0; }
+.pr-ex-title { flex: 1; }
+.pr-ex-meta { font-size: .76rem; color: var(--text-3); }
+.pr-ex-empty { padding: 16px; text-align: center; color: var(--text-3); font-size: .85rem; }