Phase 5: library & stats

- Full library home page: question list with per-user mastery stats,
  search/source filter, sort (A-Z, weakest first, most-seen), source
  breakdown in header, Take a test CTA.
- GET/POST /questions/{id}: view and edit question text, source, answers;
  radio-select correct answer; shows seen×/correct% stat.
- POST /questions/{id}/delete: hard delete (cascades to answers via FK).
- repo: ListQuestions supports SortWeakest/SortMostSeen via LEFT JOIN;
  added CountBySource, UpdateQuestion, UpdateAnswers, DeleteQuestion.
- render: added pct template func (correct*100/seen).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/db/repo.go

@@ -149,23 +149,37 @@ func (r *Repo) GetQuestion(id string) (*models.Question, []*models.Answer, error
 }
 
 func (r *Repo) ListQuestions(f ListFilter) ([]*models.Question, error) {
-	var where []string
 	var args []any
 
+	join := ""
+	if f.UserID != 0 && (f.Sort == SortWeakest || f.Sort == SortMostSeen) {
+		join = " LEFT JOIN user_question_stats s ON s.question_id = q.id AND s.user_id = ?"
+		args = append(args, f.UserID)
+	}
+
+	var where []string
 	if f.Source != "" {
-		where = append(where, "source = ?")
+		where = append(where, "q.source = ?")
 		args = append(args, f.Source)
 	}
 	if f.Search != "" {
-		where = append(where, "text LIKE ?")
+		where = append(where, "q.text LIKE ?")
 		args = append(args, "%"+f.Search+"%")
 	}
 
-	query := "SELECT id, text, source, created_at FROM questions"
+	query := "SELECT q.id, q.text, q.source, q.created_at FROM questions q" + join
 	if len(where) > 0 {
 		query += " WHERE " + strings.Join(where, " AND ")
 	}
-	query += " ORDER BY text COLLATE NOCASE ASC"
+
+	switch f.Sort {
+	case SortWeakest:
+		query += " ORDER BY CASE WHEN s.times_seen IS NULL OR s.times_seen = 0 THEN 0.0 ELSE CAST(s.times_correct AS REAL) / s.times_seen END ASC, q.text COLLATE NOCASE ASC"
+	case SortMostSeen:
+		query += " ORDER BY COALESCE(s.times_seen, 0) DESC, q.text COLLATE NOCASE ASC"
+	default:
+		query += " ORDER BY q.text COLLATE NOCASE ASC"
+	}
 
 	rows, err := r.db.Query(query, args...)
 	if err != nil {
@@ -206,6 +220,61 @@ func (r *Repo) ListSources() ([]string, error) {
 	return sources, rows.Err()
 }
 
+// SourceStat holds a source name with its question count.
+type SourceStat struct {
+	Source string
+	Count  int
+}
+
+func (r *Repo) CountBySource() ([]SourceStat, error) {
+	rows, err := r.db.Query(
+		"SELECT source, COUNT(*) FROM questions WHERE source != '' GROUP BY source ORDER BY source",
+	)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+
+	var stats []SourceStat
+	for rows.Next() {
+		var s SourceStat
+		if err := rows.Scan(&s.Source, &s.Count); err != nil {
+			return nil, err
+		}
+		stats = append(stats, s)
+	}
+	return stats, rows.Err()
+}
+
+func (r *Repo) UpdateQuestion(id, text, source string) error {
+	_, err := r.db.Exec("UPDATE questions SET text = ?, source = ? WHERE id = ?", text, source, id)
+	return err
+}
+
+// AnswerUpdate carries the fields to write for a single answer row.
+type AnswerUpdate struct {
+	ID        int64
+	Text      string
+	IsCorrect bool
+}
+
+func (r *Repo) UpdateAnswers(updates []AnswerUpdate) error {
+	for _, u := range updates {
+		if _, err := r.db.Exec(
+			"UPDATE answers SET text = ?, is_correct = ? WHERE id = ?",
+			u.Text, u.IsCorrect, u.ID,
+		); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+func (r *Repo) DeleteQuestion(id string) error {
+	_, err := r.db.Exec("DELETE FROM questions WHERE id = ?", id)
+	return err
+}
+
 func (r *Repo) CountQuestions() (int, error) {
 	var n int
 	return n, r.db.QueryRow("SELECT COUNT(*) FROM questions").Scan(&n)