feat: background tasks manager and book sentence mining for CEFR tests

app/static/css/style.css

@@ -4276,7 +4276,89 @@ body {
 }
 
 .cefr-header {
-    margin-bottom: 28px;
+    display: flex;
+    justify-content: space-between;
+    align-items: flex-start;
+    gap: 16px;
+    margin-bottom: 24px;
+    flex-wrap: wrap;
+}
+
+.cefr-header-actions {
+    display: flex;
+    align-items: center;
+    gap: 10px;
+}
+
+/* Background Task Banner in CEFR Overview */
+.cefr-task-banner {
+    background: linear-gradient(135deg, rgba(59, 130, 246, 0.12), rgba(16, 185, 129, 0.08));
+    border: 1px solid rgba(59, 130, 246, 0.3);
+    border-radius: var(--radius-md);
+    padding: 14px 18px;
+    margin-bottom: 24px;
+    box-shadow: 0 4px 12px rgba(0, 0, 0, 0.08);
+}
+
+.task-banner-inner {
+    display: flex;
+    justify-content: space-between;
+    align-items: center;
+    gap: 16px;
+}
+
+.task-banner-content {
+    flex: 1;
+}
+
+.task-banner-title {
+    display: flex;
+    align-items: center;
+    gap: 8px;
+    font-size: 14px;
+    color: var(--text-main);
+    margin-bottom: 4px;
+}
+
+.task-banner-counts {
+    font-size: 12px;
+    color: var(--text-muted);
+    font-weight: 500;
+    margin-left: auto;
+}
+
+.task-banner-msg {
+    font-size: 13px;
+    color: var(--text-muted);
+}
+
+/* Book Source Badge in Question Card & Feedback */
+.cefr-book-badge {
+    display: inline-flex;
+    align-items: center;
+    gap: 4px;
+    font-size: 12px;
+    font-weight: 600;
+    color: #3b82f6;
+    background: rgba(59, 130, 246, 0.1);
+    border: 1px solid rgba(59, 130, 246, 0.25);
+    padding: 2px 10px;
+    border-radius: var(--radius-sm);
+    max-width: 280px;
+    overflow: hidden;
+    text-overflow: ellipsis;
+    white-space: nowrap;
+}
+
+.fb-source-citation {
+    font-size: 12.5px;
+    font-weight: 500;
+    color: #3b82f6;
+    background: rgba(59, 130, 246, 0.08);
+    border-left: 3px solid #3b82f6;
+    padding: 6px 10px;
+    border-radius: 0 var(--radius-sm) var(--radius-sm) 0;
+    margin: 8px 0;
 }
 
 .cefr-title-wrap h2 {