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/cmd/controller/main.go

@@ -355,7 +355,7 @@ func main() {
 
 	// --- Auto-discover storage paths from deployed apps ---
 	discoveredPaths := discoverHDDPaths(cfg.Paths.StacksDir, logger)
-	sett.AutoDiscoverStoragePaths(discoveredPaths, cfg.Paths.HDDPath, logger)
+	sett.AutoDiscoverStoragePaths(discoveredPaths, logger)
 
 	// --- Load or create encryption key ---
 	encKeyPath := filepath.Join(cfg.Paths.DataDir, "encryption.key")
@@ -523,10 +523,7 @@ func main() {
 
 	if metricsStore != nil {
 		defer metricsStore.Close()
-		metricsHDDPath := cfg.Paths.HDDPath
-		if p := sett.GetDefaultStoragePath(); p != "" {
-			metricsHDDPath = p
-		}
+		metricsHDDPath := sett.GetDefaultStoragePath() // R-492: no global fallback any more
 		metricsCollector := metrics.NewMetricsCollector(metricsStore, cpuCollector, metricsHDDPath, logger)
 		metricsCollector.Start(ctx)
 		defer metricsCollector.Stop()