D5: an app restore works from the drive alone (v0.188.0)

The recovery unit on the customer's drive now carries the PORTABLE secret
class, so Tier-1/Tier-2 restore no longer depends on the whole-guest tier.
A customer needs the drive and nothing else.

Part 0's rulings overturned the brief's recommendation, on evidence:
- the data_key flag is untrustworthy (4+ encryption keys the catalog itself
  labels as such are unflagged) -> R-127
- a DB password is not resettable in practice: POSTGRES_PASSWORD is ignored
  once PGDATA is non-empty, so a regenerated value leaves the app unable to
  authenticate against its own restored rows while the dump replay still
  reports success (proven on a throwaway postgres:16-alpine)

Ruling (operator): type:secret travels, type:password never does, minus the
nonPortableSecrets code register. Plaintext -- withholding the internet-
reachable class is what licenses that, and the two are coupled.

Precedence: the UNIT WINS over the guest -- the unit's secrets were captured
in the same run as the dumps beside them, so they match the data being
restored. The fail-closed data-key gate is unchanged.

Secret values are never logged; the manifest records NAMES only.

controller/internal/appbackup/paths.go

@@ -40,9 +40,10 @@ func PrimaryBackupPath(nsRoot string) string {
 // RecoveryUnitPath returns the per-app self-contained recovery-unit ROOT under a namespace root.
 // It is the existing per-app backup dir (`backups/primary/<stack>/`) — the legacy name is kept so the
 // db-dumps/ and volume-dumps/ already written there need no migration; the unit gains compose/ and
-// manifest.json as siblings, making the whole dir a complete, recreatable unit (Phase 2). The unit is
-// secret-free: secrets/data-keys are recovered from the guest's own app.yaml (live or via PBS), never
-// stored here. See backup.recoveryUnit / restore for the capture + restore flow.
+// manifest.json as siblings, making the whole dir a complete, recreatable unit (Phase 2). Since D5 the
+// unit's compose/app.yaml CARRIES the portable secret class (data keys, DB passwords, internal signing
+// secrets) at mode 0600, so a Tier-1/2 restore needs the drive and nothing else; internet-reachable
+// admin logins are still withheld. See backup.recoveryUnit / restore for the capture + restore flow.
 func RecoveryUnitPath(nsRoot, stackName string) string {
 	return filepath.Join(nsRoot, "backups", "primary", stackName)
 }