v0.54.0: Phase 2b — restore-from-recovery-unit + fail-closed data-key gate

Restore recreates an app from its on-drive unit + the guest's own secrets,
regenerating nothing. reconcileRestoreSecrets (pure, unit-tested) merges the unit's
non-secret env with secrets recovered from the live app.yaml and FAILS CLOSED if a
data-encrypting key is unrecoverable (refuse — a PBS whole-guest restore is needed —
rather than regenerate and corrupt). Resettable secrets missing → warn + proceed.

- backup: RestoreFromRecoveryUnit (manifest -> recover secrets -> gate -> restore
  volumes -> recreate definition + redeploy w/ re-pull); falls back to volume-only.
- seams: RecoverStackSecrets/RecreateStackFromUnit (adapter +encKey),
  stacks.RedeployFromEnv. Wired into /backup/restore.
- tests: gate (refuse/proceed/verbatim) + data_key parsing.

Gate + reconcile + data_key parsing unit-tested; capture live-validated (v0.53.1).
Full readable-data e2e vs AdventureLog needs the auth-gated dashboard restore — pending.

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

controller/internal/web/handlers.go

@@ -714,7 +714,9 @@ func (s *Server) backupRestoreHandler(w http.ResponseWriter, r *http.Request) {
 	s.logger.Printf("[WARN] [web] Restore requested: stack=%s, snapshot=%s from %s", stackName, snapshotID, r.RemoteAddr)
 
 	start := time.Now()
-	err := s.backupMgr.RestoreApp(stackName, snapshotID)
+	// Phase 2b: restore from the app's recovery unit (recovers secrets from the guest, fail-closed on
+	// an unrecoverable data-encrypting key; falls back to volume-only restore if no unit exists).
+	err := s.backupMgr.RestoreFromRecoveryUnit(stackName)
 	if err != nil {
 		s.logger.Printf("[ERROR] [web] Restore failed: %v", err)
 		if s.isDebug() {