Stage 17 (#15): admin users people/robots toggle + display-name & external-id glob filters

- account.ListUsers/CountUsers with a UserFilter: people vs robots (by a robot identity),
  case-insensitive '*'/'?' glob masks on display_name and any identity's external_id
- admin users list shows the real kind (robot/guest/registered), defaults to people,
  with a People/Robots toggle + a filter form; pager preserves the filter
- integration test for the filter; SQL verified against the live contour DB

backend/internal/account/userlist.go

@@ -0,0 +1,95 @@
+package account
+
+import (
+	"context"
+	"fmt"
+	"strings"
+	"time"
+
+	"github.com/google/uuid"
+)
+
+// UserListItem is the admin user-list projection: a small subset of the account plus
+// whether it is a robot (derived from its identities), so the console can label the kind
+// without a per-row identity query.
+type UserListItem struct {
+	ID                uuid.UUID
+	DisplayName       string
+	PreferredLanguage string
+	IsGuest           bool
+	IsRobot           bool
+	CreatedAt         time.Time
+}
+
+// UserFilter narrows the admin user list: Robots selects robot accounts (otherwise the
+// non-robot "people"); NameMask and ExternalIDMask are glob masks ('*' = any run, '?' =
+// one char) matched case-insensitively against the display name / any identity's external
+// id. An empty mask means no filter on that field.
+type UserFilter struct {
+	Robots         bool
+	NameMask       string
+	ExternalIDMask string
+}
+
+// robotExists is the correlated subquery testing whether account a is a robot.
+const robotExists = `EXISTS (SELECT 1 FROM backend.identities i WHERE i.account_id = a.account_id AND i.kind = 'robot')`
+
+// userListWhere builds the shared WHERE clause and its positional args (from $1).
+func userListWhere(f UserFilter) (string, []any) {
+	args := []any{f.Robots}
+	where := robotExists + ` = $1`
+	if name := likePattern(f.NameMask); name != "" {
+		args = append(args, name)
+		where += fmt.Sprintf(` AND a.display_name ILIKE $%d ESCAPE '\'`, len(args))
+	}
+	if ext := likePattern(f.ExternalIDMask); ext != "" {
+		args = append(args, ext)
+		where += fmt.Sprintf(` AND EXISTS (SELECT 1 FROM backend.identities i WHERE i.account_id = a.account_id AND i.external_id ILIKE $%d ESCAPE '\')`, len(args))
+	}
+	return where, args
+}
+
+// ListUsers returns the filtered admin user list, newest first, paginated.
+func (s *Store) ListUsers(ctx context.Context, f UserFilter, limit, offset int) ([]UserListItem, error) {
+	where, args := userListWhere(f)
+	q := `SELECT a.account_id, a.display_name, a.preferred_language, a.is_guest, a.created_at, ` + robotExists + ` AS is_robot
+FROM backend.accounts a WHERE ` + where +
+		fmt.Sprintf(` ORDER BY a.created_at DESC LIMIT $%d OFFSET $%d`, len(args)+1, len(args)+2)
+	args = append(args, limit, offset)
+	rows, err := s.db.QueryContext(ctx, q, args...)
+	if err != nil {
+		return nil, fmt.Errorf("account: list users: %w", err)
+	}
+	defer rows.Close()
+	var out []UserListItem
+	for rows.Next() {
+		var it UserListItem
+		if err := rows.Scan(&it.ID, &it.DisplayName, &it.PreferredLanguage, &it.IsGuest, &it.CreatedAt, &it.IsRobot); err != nil {
+			return nil, fmt.Errorf("account: scan user: %w", err)
+		}
+		out = append(out, it)
+	}
+	return out, rows.Err()
+}
+
+// CountUsers counts the filtered admin user list, for pagination.
+func (s *Store) CountUsers(ctx context.Context, f UserFilter) (int, error) {
+	where, args := userListWhere(f)
+	var n int
+	if err := s.db.QueryRowContext(ctx, `SELECT COUNT(*) FROM backend.accounts a WHERE `+where, args...).Scan(&n); err != nil {
+		return 0, fmt.Errorf("account: count users: %w", err)
+	}
+	return n, nil
+}
+
+// likePattern converts a glob mask ('*' any run, '?' one char) to an ILIKE pattern,
+// escaping the SQL wildcards already in the input first. An empty/blank mask returns "".
+func likePattern(mask string) string {
+	mask = strings.TrimSpace(mask)
+	if mask == "" {
+		return ""
+	}
+	escaped := strings.NewReplacer(`\`, `\\`, `%`, `\%`, `_`, `\_`).Replace(mask)
+	escaped = strings.ReplaceAll(escaped, "*", "%")
+	return strings.ReplaceAll(escaped, "?", "_")
+}