Default to 8-char slugs for public-internet-safe entropy

5-char slugs over a 32-char alphabet are ~25 bits; brute-forceable at
HTTP speeds once the endpoint is public. 8 chars = 40 bits (~10^12
combinations), infeasible to scan within useful timescales. Existing
5-char slugs still resolve; only new artifacts use the longer form.

internal/store/artifacts.go

@@ -38,7 +38,7 @@ type CreateOptions struct {
 func (s *Store) CreateArtifact(opts CreateOptions) (*Artifact, error) {
 	slug := strings.ToLower(strings.TrimSpace(opts.Slug))
 	if slug == "" {
-		g, err := s.generateSlug(5)
+		g, err := s.generateSlug(8)
 		if err != nil {
 			return nil, err
 		}