B1: data-migration engine (MigrateAll + MigrateApp) + backup mutual-exclusion

internal/stacks/migrate.go: crash-safe, resumable namespace migration over the
controller's /mnt RW mount. Two entry points (whole-namespace + per-app) share one
journaled pipeline: validate -> stop -> copy (rsync -a --checksum, additive; conflict-
merge walk for non-app content) -> verify -> flip+redeploy (RedeployFromEnv) -> cleanup.
CLEANUP (the only destructive step) is gated on all units verified AND all apps
redeployed. Single-flight; mutual exclusion with the backup orchestrator (Change 3).
Non-hollow tests incl. mutation-proven collision + cleanup-gate companions.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

controller/internal/backup/backup.go

@@ -36,6 +36,11 @@ type Manager struct {
 	discoverDBs  func(ctx context.Context) ([]DiscoveredDB, error)
 	importDBDump func(ctx context.Context, db DiscoveredDB, dumpPath string) error
 
+	// migrationRunning, if set, reports whether a data migration is in progress. The scheduled
+	// backup paths skip when it returns true (Change 3 — backup ↔ migration mutual exclusion), so a
+	// nightly dump/Tier-2 can't race a migration copy/cleanup on the same drive.
+	migrationRunning func() bool
+
 	mu         sync.Mutex
 	lastDBDump *DBDumpStatus
 	running    bool
@@ -158,8 +163,23 @@ func (m *Manager) groupStacksByDrive() map[string][]StackSummary {
 	return result
 }
 
+// SetMigrationRunningCheck wires the mutual-exclusion guard (Change 3): when fn() reports a
+// migration is active, the scheduled backup paths skip rather than race it.
+func (m *Manager) SetMigrationRunningCheck(fn func() bool) {
+	m.migrationRunning = fn
+}
+
+// migrationActive reports whether a migration is in progress (false when no check is wired).
+func (m *Manager) migrationActive() bool {
+	return m.migrationRunning != nil && m.migrationRunning()
+}
+
 // RunDBDumps discovers and dumps all databases to per-drive, per-app paths.
 func (m *Manager) RunDBDumps(ctx context.Context) error {
+	if m.migrationActive() {
+		m.logger.Printf("[INFO] [backup] DB dump kihagyva: migráció folyamatban")
+		return nil
+	}
 	if err := m.acquireRunning(); err != nil {
 		return err
 	}