Address production-readiness review: clearer errors, safer votes, and ops hardening.

Distinguish auth/lookup failures, make votes idempotent on visible questions, bound shutdown, page admin users, LRU throttle, trusted-proxy CIDRs, avatar cleanup, versioned migrations, and session cleanup logging.

internal/store/memory.go

@@ -43,7 +43,7 @@ func (m *Memory) CreateUser(_ context.Context, u *User) error {
 	}
 	u.Username = NormalizeUsername(u.Username)
 	if _, ok := m.byName[u.Username]; ok {
-		return fmt.Errorf("username taken")
+		return ErrDuplicateUsername
 	}
 	if u.ID == "" {
 		u.ID = uuid.NewString()
@@ -94,18 +94,44 @@ func (m *Memory) UserByUsername(_ context.Context, username string) (*User, erro
 	return &cp, nil
 }
 
-func (m *Memory) ListUsers(_ context.Context) ([]User, error) {
+func (m *Memory) ListUsers(_ context.Context, q ListUsersQuery) ([]User, string, string, error) {
 	m.mu.Lock()
 	defer m.mu.Unlock()
+	limit := q.Limit
+	if limit <= 0 {
+		limit = AdminUsersLimit
+	}
+	search := strings.ToLower(strings.TrimSpace(q.Search))
 	out := make([]User, 0, len(m.users))
 	for _, u := range m.users {
+		if search != "" &&
+			!strings.Contains(strings.ToLower(u.Username), search) &&
+			!strings.Contains(strings.ToLower(u.Name), search) {
+			continue
+		}
+		if q.CursorCreated != "" {
+			if u.CreatedAt > q.CursorCreated {
+				continue
+			}
+			if u.CreatedAt == q.CursorCreated && u.ID >= q.CursorID {
+				continue
+			}
+		}
 		out = append(out, *u)
 	}
-	sort.Slice(out, func(i, j int) bool { return out[i].CreatedAt < out[j].CreatedAt })
-	if len(out) > AdminUsersLimit {
-		out = out[:AdminUsersLimit]
+	sort.Slice(out, func(i, j int) bool {
+		if out[i].CreatedAt != out[j].CreatedAt {
+			return out[i].CreatedAt > out[j].CreatedAt
+		}
+		return out[i].ID > out[j].ID
+	})
+	var nextCreated, nextID string
+	if len(out) > limit {
+		last := out[limit-1]
+		nextCreated, nextID = last.CreatedAt, last.ID
+		out = out[:limit]
 	}
-	return out, nil
+	return out, nextCreated, nextID, nil
 }
 
 func (m *Memory) CountAdmins(_ context.Context) (int, error) {
@@ -331,16 +357,17 @@ func (m *Memory) UpsertAnswer(_ context.Context, a *Answer) error {
 func (m *Memory) Vote(_ context.Context, userID, questionID string, value int) error {
 	m.mu.Lock()
 	defer m.mu.Unlock()
-	if value != 1 && value != -1 {
+	if value != 1 && value != -1 && value != 0 {
 		return fmt.Errorf("invalid vote")
 	}
-	if _, ok := m.questions[questionID]; !ok {
-		return sql.ErrNoRows
+	q, ok := m.questions[questionID]
+	if !ok || q.Hidden {
+		return ErrHiddenOrMissing
 	}
 	if m.votes[questionID] == nil {
 		m.votes[questionID] = map[string]int{}
 	}
-	if cur, ok := m.votes[questionID][userID]; ok && cur == value {
+	if value == 0 {
 		delete(m.votes[questionID], userID)
 		return nil
 	}