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/config/config.go

@@ -164,7 +164,6 @@ type PathsConfig struct {
 	StacksDir      string `yaml:"stacks_dir"`
 	DataDir        string `yaml:"data_dir"`
 	SystemDataPath string `yaml:"system_data_path"`
-	HDDPath        string `yaml:"hdd_path"`
 }
 
 type WebConfig struct {
@@ -450,7 +449,6 @@ func applyEnvOverrides(cfg *Config) {
 	envStr("FELHOM_WEB_LISTEN", &cfg.Web.Listen)
 	envStr("FELHOM_WEB_PASSWORD_HASH", &cfg.Web.PasswordHash)
 	envStr("FELHOM_PATHS_STACKS_DIR", &cfg.Paths.StacksDir)
-	envStr("FELHOM_PATHS_HDD_PATH", &cfg.Paths.HDDPath)
 	envStr("FELHOM_LOGGING_LEVEL", &cfg.Logging.Level)
 	envStr("FELHOM_MONITORING_SYSTEM_HEALTH_INTERVAL", &cfg.Monitoring.SystemHealthInterval)
 }