fix(api): return empty arrays not null for empty lists (frontend null.length crash)

Root cause: store List* methods used 'var out []T' which stays nil on empty
result sets and serializes to JSON null; the SPA then crashed on .length/.map
(e.g. endpoints.length on the Tasks page right after deploy). Return []T{}
at the source; coerce null->[] on the frontend load sites as defense-in-depth.
Regression test asserts List* are non-nil when empty.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MMHQTtnQtQqL8muAXHr9kd

internal/store/endpoints.go

@@ -34,7 +34,7 @@ func (s *Store) ListEndpoints(ctx context.Context) ([]Endpoint, error) {
 		return nil, err
 	}
 	defer rows.Close()
-	var out []Endpoint
+	out := []Endpoint{}
 	for rows.Next() {
 		var e Endpoint
 		if err := rows.Scan(&e.ID, &e.RoleLabel, &e.Host, &e.Port, &e.TLSMode); err != nil {