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/migrate.go

@@ -13,6 +13,7 @@ import (
 	"sync"
 	"time"
 
+	"gitea.dooplex.hu/admin/felhom-controller/internal/backup"
 	"gitea.dooplex.hu/admin/felhom-controller/internal/settings"
 )
 
@@ -394,8 +395,8 @@ func (o *MigrateOrchestrator) RunEnhancedMigration(
 	// --- Post-migration steps (all non-fatal) ---
 
 	// 1. Copy DB dumps from source to destination
-	srcDBDumps := filepath.Join(req.CurrentHDDPath, "backups", "primary", req.StackName, "db-dumps")
-	dstDBDumps := filepath.Join(req.TargetPath, "backups", "primary", req.StackName, "db-dumps")
+	srcDBDumps := backup.AppDBDumpPath(req.CurrentHDDPath, req.StackName)
+	dstDBDumps := backup.AppDBDumpPath(req.TargetPath, req.StackName)
 	if _, err := os.Stat(srcDBDumps); err == nil {
 		if err := os.MkdirAll(filepath.Dir(dstDBDumps), 0755); err != nil {
 			o.Logger.Printf("[WARN] Migration %s: failed to create DB dump dir: %v", req.StackName, err)