Implement time off management (Issue #3)

Add full time-off lifecycle: create/edit/delete with shift conflict
detection, auto-removal from conflicting shifts with admin notification,
shift restoration on admin delete, and hard block on assigning volunteers
with approved time off to shifts.

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

internal/volunteer/volunteer.go

@@ -397,6 +397,25 @@ func (s *Store) RecordLogin(ctx context.Context, id int64) error {
 	return err
 }
 
+// ListAdminIDs returns the IDs of all active admin users.
+func (s *Store) ListAdminIDs(ctx context.Context) ([]int64, error) {
+	rows, err := s.db.QueryContext(ctx,
+		`SELECT id FROM volunteers WHERE role = 'admin' AND active = 1`)
+	if err != nil {
+		return nil, fmt.Errorf("list admin IDs: %w", err)
+	}
+	defer rows.Close()
+	var ids []int64
+	for rows.Next() {
+		var id int64
+		if err := rows.Scan(&id); err != nil {
+			return nil, err
+		}
+		ids = append(ids, id)
+	}
+	return ids, rows.Err()
+}
+
 func generateToken() (string, error) {
 	b := make([]byte, 32)
 	if _, err := rand.Read(b); err != nil {