v0.26.0: Storage namespace felhom-data/ + test node wipe script

All felhom-managed data on external drives now lives under felhom-data/
subdirectory, cleanly separating controller data from user files.

- backup/paths.go: add FelhomDataDir constant, update 8 path helpers
- stacks/delete.go: add local felhomDataDir constant (circular import
  boundary), update ProtectedHDDPaths + GetStackBackupData
- storage/migrate_drive.go: import backup pkg, fix conflict check, verify,
  rsync excludes (felhom-data/backups/*/restic/), size estimation
- storage/migrate.go: import backup pkg, fix DB dump paths
- web/handlers.go: fix legacy 'storage' path -> backup.AppDataDir()
- storage/format_linux.go: create felhom-data/ instead of storage/
- storage/attach_linux.go: create felhom-data/ instead of storage/
- scripts/felhom-wipe.sh: new multi-level test node wipe script
  (soft/controller/full/nuclear)
- CHANGELOG.md, controller/README.md, scripts/README.md: updated docs

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

controller/internal/storage/format_linux.go

@@ -208,7 +208,7 @@ func FormatAndMount(req FormatRequest, progress chan<- FormatProgress) (string,
 
 	_ = exec.Command("chown", "1000:1000", mountPath).Run()
 
-	for _, subdir := range []string{"storage", "Dokumentumok"} {
+	for _, subdir := range []string{"felhom-data", "Dokumentumok"} {
 		dir := filepath.Join(mountPath, subdir)
 		if err := os.MkdirAll(dir, 0755); err == nil {
 			_ = exec.Command("chown", "1000:1000", dir).Run()