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/web/handlers.go

@@ -1220,7 +1220,7 @@ func (s *Server) appDetailsForPath(storagePath string) []StorageAppDetail {
 			Stack: stack.Meta.Slug,
 		}
 		// Try to get data size from the storage subdirectory
-		appDataDir := filepath.Join(storagePath, "storage", stack.Name)
+		appDataDir := backup.AppDataDir(storagePath, stack.Name)
 		if fi, err := os.Stat(appDataDir); err == nil && fi.IsDir() {
 			detail.SizeHuman = dirSizeHuman(appDataDir)
 		}