R-492: delete the always-empty cfg.Paths.HDDPath (field, env binding, every fallback)

Readers (report builder, health check, /api/system, web primaryHDDPath, metrics collector,
AutoDiscoverStoragePaths' fallback parameter) now use the storage registry only. An old
controller.yaml still carrying paths.hdd_path keeps loading (non-strict YAML), pinned by
TestR492_OldConfigWithHDDPathStillLoads.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0159rPz1ZhFKsS53msqPYxtS

controller/internal/web/server.go

@@ -982,12 +982,10 @@ func (s *Server) findStackBySubdomain(subdomain string) (*stacks.Stack, bool) {
 	return nil, false
 }
 
-// primaryHDDPath returns the default storage path, or the legacy config value.
+// primaryHDDPath returns the default storage path ("" when none is registered — R-492 deleted the
+// always-empty legacy config value it used to fall back to).
 func (s *Server) primaryHDDPath() string {
-	if p := s.settings.GetDefaultStoragePath(); p != "" {
-		return p
-	}
-	return s.cfg.Paths.HDDPath
+	return s.settings.GetDefaultStoragePath()
 }
 
 func (s *Server) render(w http.ResponseWriter, name string, data interface{}) {